Isn't it obvious ? Man, just think here. Who own that much token ? Who could afford that kind of dump ?
We got two options:

A - The dev is dumping
B - A whale is playing with the price

I go for option A; Why ? Because today the platform should launch but, if you know there is no platform., then you know that people will dump and the price will crash... So you anticipate that and you dump your token to get the maximum profit out of your "project"