First off my condolences.. I know my pets are like my kids to me..

now as far as the reason goes.....its possible.. but honestly the only way you would know if you had taken the dog to the vet and had a necropsy (autopsy) done.. She was in the garage.. so she could have suffered from heatstroke...or could have had an underlying heart problem...or god forbid as someone said before she could have somehow gotten into something that she ate that cause her death. It's very hard to tell..

We had a lady come into the office two days ago (i work for a vet up here in braselton) and her little pug she had only had for less than a week had gotten out of the house. She found it the next day under her deck and notice the dog was having major trouble breathing.. she brought it in, but sometime during her finding the dog and getting to the vet, the pup had died. The doctor found lots of bite marks on the underside of the pup.. and the owner said she never saw them.. I didnt tell you this to depress you.. just saying crazy things can happen...