Am I liable for a pets actions when I am not there?

I just recently did a request that was just under 10 days and I always notify the owners that I do have a full time job and I go to school so I like to make sure their dog will be ok if left alone for about 8-10 hours depending on how far they are from my job, and these customers were ok, now they are back and they are accusing me of neglecting their pets for not being at the house because it was not clean and they found mold on the carpet, but when I left the house that morning before work the house looked like when I had gotten there. I looked it up and mold doesn't become visible anywhere from 12 days-5weeks and the dog ripped up the mail and some boxes they received, I placed them where they told me to, and I didn't see her chew up the mail or else I would have stopped it. And she didn't do it until I had left the last day and my instinct says that she got really anxious because her owners have been gone for an extended period of time and before that they were getting married so probably weren't around too much for that and the last straw was her seeing me leaving with my duffel and backpack. I don't know how to handle this because I preformed my duties as I had said I would, i was even late to work because I wanted to make sure the dog let out some energy before I left and went to the bathroom. Anyone has any advise?