It used to be the case that the 2 or more housemates that had the most votes were put up for eviction but now it seems to be anyone who gets more than one vote or something.

It no longer seems to be that if one housemate gets 5 nominations, the second highest 4, with the rest lower than that all escape being up, it looks anyone who gets more than one nomination is put up for eviction. Am I correct? If I am, when and why did they change this? To make more money?

There is no rule anymore, they just make it up as they go along.

Yes they change it every week

Jackie Travers found herself up with only 2 votes as have a fair few HMs in BB15 and the Celev BBs. They do whatever they want these days

They do seem to vary it actually but it appears they prefer more than two up nowadays

They wanted jack up as well as they think it will help draw eviction votes away from big marq.

more up = more money spent. it's a commercial decision to make a bit more cash from the mugs.