You don't sell Bitcoins 1 day before IPO and also IPO is not for small guys. Bullshit mostly this as a reason, this was purely momentum triggered after nice work done the same way as many times before from the same wall-bear. But could still be some guys selling for IPO, but nothing spectacular.
Well, Alibaba is Chinese, and it seems that the price only reacts to news that matter to the Chinese..
Can we tell which exchanges are leading the drop?
IMHO, the coincidence of times justifies looking for a connection between the drop and the IPO.
For instance, could it be that Chinese hoarders (not traders) moving their off-exchange coins to the "Western" exchanges and selling them there, in a hurry, in order to obtain dollars for Alibaba's IPO?