Advice wanted

My new dog is charming and sweet in every way but one -- when I put her in her crate because I have to leave the house, she invariably pees. This is bass-ackward, because conventional wisdom is that crates are the one place in which a dog won't pee. She also sleeps in the crate at night, and doesn't soil it then. She only does so when I leave her alone in the house. This holds true despite the fact that I walk her before I crate her, and I seldom leave her in the crate for more than an hour or two. I can't decide if she's over-excited, stressed or vindictive. Any advice for dealing with this problem?