Why φυγόντες and not φύγοντες?
hi, see chandler s779 on 2nd aorist active ppls:
http://archive.org/stream/accentuationgree00chanuoft#page/220/mode/2up
cheers, chad 
Or another answer is, “Because that’s how Greek children heard their parents pronouncing it, when they were learning how to talk!”
Probably not what you were looking for, but true nonetheless