Depends what caused the haematoma in the first place. There are various causes. The most obvious is that a small blood vessel that should have been cauterized wasn't, or it was but it 'blew' the clot off the cut end. Not many people realise that tissue can ooze a lot after surgery. This is nothing to do with actual bleeding, but you know how a burn can weep? Or a blister? It's pretty much like that. Usually, after hip surgery, the surgeon puts vacuum drains in the wound to stop haematomas developing and for the same reason, if they have evacuated it, they usually put a vacuum drain in the wound to prevent it happening again. In your shoes, I would express concern if they have not done this.