Probably it is a Bot. The transaction is too fast for you to see. There is also a possibility that exchange is manipulating the sales but it is more probably the owner of the token you are trading. I also experience this kind of stuff but I just ignore it since I cannot do anything about it. I just set my sell order and leave it until the order is fulfilled.
Probably, but this let me ask myself: why? What did they win with that?
Maybe there is a pump-and-dump group behind using a bot trying to manipulate the price. And tell their members that they are indeed can control the price of this coin. Just simply some preparation for them to make it looks like they've holding majority of this coin and then one day push the price again followed by a big dump. Or just exchanges as well, its possible because we don't see what's going on from behind. There's a lot of things that can really happen that is hard to explain. You can event suspect that the project itself is doing some manipulation as well. There are so many angles to look, IMHO.