I have had it happen prolly about 5 times in EB. Usually you can tell where this is most likely to happen if you look around; frex, if you ally with the Ptolies or AS, look to ttheir mutual border and park some units nearby. If you wander enough, you can generally assist your ally.
Most often though I get the computer aiding me. In fact in one of my old Romani campaigns I actually triggered the Lusotann/Carthage war by getting the Luso's to aid me in a battle.