there is no need for all exchanges to have the exact same price at all times all day long. they fluctuate differently based on the buys/sells that happen on each exchange almost separately.
this difference you are pointing out is just a small difference. although looking at the price i see 0.124BTC on bittrex so it probably dropped there too but with a little bit of delay.

and remember that poloniex being bigger means it also has a lot more weak hands that contribute more to the wave of panic sells that crashes the price faster. then other exchanges follow.