Peeing in the crate

We recently brought home a small mixed breed pup to our home. She is now roughly 6 months old. We got her when she was 8 weeks. She has done well with the potty training and only had accidents, for the most part, when me and my girlfriend were slacking on bringing her outside. Just recently she has started peeing in her crate right when we get home. She does fine in the crate for around 4-5 hours straight. She is in there in the morning when my girlfriend and I are at work. She comes home on lunch and lets her out. Then she is back in the crate for the rest of the work day. When we come home at the end of the day or from a trip out on the weekend she waits until we are about to let her out of the crate and then she pees. She hasn't been doing this very long, but it has quickly gotten old. Do any of you have any tips for breaking this behavior? Like I said, she is fine for long stretches in the crate and she is fine all through the night when she sleeps in our bed with us. It feels like a defiant behavior that she recently developed concerning the crate. Also, important to note, when ever she is going to be put in the crate she is taken out side and given time to do her business before she is crated.

So far we have tried: Waiting to let her out when we get home. Immediately giving her attention and letting her out. Giving her a treat before we open the door to let her out. Nothing has worked. She is not peeing out of excitement, at least it doesn't look like that. I have never had a small dog and I have never had a female dog. I am at the end of my rope with this behavior. She begins obedience training next weekend but I need to keep trying to solve this problem in the mean time.

Thanks.