About a month ago, Abbey did the exact same thing--she had an accident in the house (on the floor), and the next morning, she went on my bed, while I was watching her, not 20 minutes after we had been out.
The first thing I would do is go to the vet to rule out a possible urinary tract or bladder infection--that's what Abbey had--just a hint of an infection, she took some antibiotics, and she's fine now.
If the vet doesn't find anything, and there's been nothing new going on (even a new comforter makes Kito go on the bed sometimes--or one fresh out of the wash), you will probably have to re-train Bull.
Good Luck