I think it was dodgy dealings, and revenge personally!
Don't know enough, but from what I hear they were in fiancial ruin anyway, and were heading for a crisis of some sort. Whether this was someone else, or a cover up I suppose it will come out eventually.....
I'm thinking murder/suicide by the father...........I have no idea why I think that but the fact the doors and windows were barricaded from the inside is very suspicious!