I do know from the Wars of the Roses that a line did collapse from the rear. It's pretty logical; you see men dying in front of you and behind you there is nothing that can harm you. it's an easy way for a coward to escape death and once he runs the guy in front of him started to worry since his backup is gone. More and more men in the backranks flee and the whole line collapses. The generals in the Wars of the Roses knew this and encouraged the infantry to stay by putting some horsemen behind them.

And about the overhand/underhand use of spears. Athletic spear throwing is done overhand. I have done it too and you can build up alot of thrust even when standing still. And the power is not only coming from the muscle arms (as someone said), you also add extra momentum by turning your torso. The only downside of overhand trusting is the extra strain of constantly holding up the spear.

Then there were also the late medieval pike units who used their pike at shoulder level or sometimes even higher. Although of course they had the advantage of using both hands making it less tiring.