Could be a fund manager trying to unload a "mistake." Yeah, it comes up on computer models as a good buy - because of book value and other fundamentals. But, if no takeover to occur, it could be dead money.
On the other hand, it could be a takeover artist trying to demonstrate that that his interest is not ONE DIMENSIONAL. That is, buy a lot and sell off substantial to drive the price down. Then, those who want to "kiss their money HELLO" will sell on the next rally - thinking that is all they are going to get. So, maybe fishing for STOP LOSSES, especially if the buyer could be a takeover artist.
But, remember, if 5% of a stock is purchased, it must be disclosed. No disclousre yet! So, NOT A TAKEOVER SO FAR. Just perhaps someone raising a flag to see if anyone salutes.