Please Help! Rottie at Shelter Snapped at Me!?

Hi there,

So I had a little bit of an issue today at the shelter where I work, with our Rottie/Hound mix. I was taking her for a walk in the park - she's extremely good on a leash and always gets really excited when going for a walk (she hops around and shakes her butt up in the air). When she calmed down, I took her for a nice long walk; we jogged for a while, I gave her some water, let her sniff around, and go to the bathroom.

The problem arose when we got back to the cage. I calmly took her back into her kennel (where she resisted a little bit; also all of the other dogs were excited and barking. So then I closed the door to the kennel, sat down and began to take off her leash. That must have triggered something in her because she just snapped at me really loudly and gnawed my arm slightly. She actually didn't even really bite me, she just put my hand in her mouth but then realized what she was doing and then stopped.

I guess what I'm wondering is what did I do wrong, and what can I do to prevent snapping in the future? I definitely want to break her of that habit before someone adopts her. I would say that I am a really calm person and assume that I usually provide that type of energy towards the dogs that I interact with, which is why I'm surprised that she snapped at me the way she did. Should I have waited until the dogs stopped barking to remove the leash? I'm just not sure what I did wrong! It's never happened to me before - this is the first time she's done it.