The Wonders a Coalition can do.

I was playing the Teutonic Order in the XL mod in High/Normal/Conquest and the Turks had conquered the Byzantines (although they still had Rhodes with three full armies), the Bulgarians, the Serbs and the Golden Khanate. They were the richest, strongest and most advanced faction and no one could hope to stand up to them. The Hungarians seemed doomed when the Sultan crossed the Danube with his armies into Hungary. The Hungarians, outnumbered 5 to 1, decided they couldn't stand a chance and abandoned Hungary. Lesser Poland, which I had conquered, was now under threat from the Turk's attack. I decided that I could not just stand by and watch a fellow Christian ally swept from the map. I shipped my full army across the Black Sea and onto Constantinople, which was poorly garrisoned. Every single troop I had was rushed into Lesser Poland. In all I had about six hundred men to protect my recruiting base from an attack of perhaps 3000 troops. I recalled my six star general from Switzerland (civil wars are terrible things you know, and I just cannot tolerate rebel territory bordering my own) with his army. And then.........the Polish reappeared in guess where, Lesser Poland. At first I was horrified but then I realised that it wasn't such a bad thing after all. I might lose a province but I gain an extra layer between me and the Sultan. I pulled out all my troops before the turn ended and destroyed the castle there, since I don't want them to be too succesful. Elsewhere, the Turks left Constantinople to me without a fight and without even withdrawing to the fortress. I guess they think they can't possibly let anything happen to such an advanced province, which is their loss. The following turn I allied myself with the Poles, the Hungarians counterattacked through Hungary into Bulgaria, the English launched a Crusade on Turkish Carpathia, Venice invade Croatia, the Crusader States took Edessa. Then the Sultan recaptured Hungary, but lost Carpathia when the Cumans re-emerged. Surrounded on all sides the only thing that could happen did: the Poles invaded Hungary. The Sultan was defeated and ransomed back. The Turkish nobles and generals could just about stomach the loss of Bulgaria and Croatia. However, my victory over the Sultan in Trebizond when I launched an amphibious attack and my joint invasion and capture of Nicaea with the Byzantines they could not swallow. The death knell for an already weakened yet not defeated nation sounded: civil war. Greece, Rum and Anatolia became rebel. To make mattters worse/better, the Golden Khan reappeared, taking Eastern Khazar, Georgia and Armenia. Only Crete and Lesser Armenia were now Turkish. The latter was gobbled up by the Crusader States. So in a last dash of bravery, the Sultan landed in Nicaea, desperately trying to gain a foothold. The Byzantines retreated to their Citadel and the next turn my armies sallied forth and gave the Province back to them. It was however, not in vain. I captured the Sultan and 2 of his heirs, who I ransomed back for the modest sum of 25000 florins. Now only tons of cash can help them now.
Never before have I seen such unity in MTW. At one point, the Venetians, the Hungarians, the Cumans, the Polish, the English, Me, the Crusader States, the Byzantines and the Golden Horde all fought the Sublime Porte. I was the primary winner, gaining Constantinople, Trebizond, Rum and Greece. The Hungarians, although capturing Bulgaria and its Fortress, lost Hungary and Carpathia. The Poles won their independance ( I would have fought them to the bitter end in other circumstances) and Hungary. The CS got Edessa and Lesser Armenia. The Byz were back in business and the Horde was a player again. Lesser winners were England and Venice, respectively getting Carpathia and Croatia. The one losser of the Coalition were the Cumans, were ground into the earth by the English crusade.
The Moral of this: no matter how pwerful you are, if your neighbours unite, you're screwed.

I call this 'the pile on syndrome' whereby once one faction attacks and wins a battle, everyone else decides the pickings will be easy and of course they are when 6 nations decide to attack one. This happens in my games all the time. The most classic case is what happens to the HRE almost every game. The Mongol Horde often succumbs to the same tactic (of course it is their own fault for attacking everyone). I make it a high priority to make the most of these situations.

As Poland I tend to take on people who already have problems. "So you want to start shit with my friends the Holy Roman Empire" Works well at the start when you are small. After awhile people start to back you up.

I too have seen this phenomenon a number of times (both as its benefactor and its victim, I might add). If someone manages to win even one major battle over a superpower, other nations will often "dogpile" onto that faction as well. If the Big Guy suffers even one defeat, often enough that's all the encouragement lesser kingdoms need to go on the offensive and take him down.