Well we had this problem a couple months ago. I don't know why, but it just started randomly happening... I'm away at college, so I would take what I have on the bed off. Put new on, take it home on weekends and wash it... But it happened for like a month straight every week. I would wake up in the middle of the night to a nice wet warm sensation thinking that I did it... haha
After about a month it stopped and so far so good. I don't know what the deal was. It only happened in the middle of the night, and while they were home alone. So we stopped letting them drink before bed, and I guess they were just mad cause we left them and was peeing on the bed to prove it.
It did happen while vacationing with my family this summer in the RV. Bentley was only like 7 months old and I know he was totally worn out by the end of the night... He was even so worn out one day he decided to pee on my sister green pillow (it did have grass like tassels... so I counted it as he thought it was grass
).