Flat tire...what happened?

Ok...just finished my first triathlon ever, a sprint tri, last weekend. Weather was great, no panic in the swim, placed 1st female novice. Can't wait for my second tri, an Olymic distance in September. So maybe this is a dumb question, but I'd like to figure out what happened with my tire. Finished the swim, ran to transition, hopped on the bike and immediately realized my front tire was flat. By that time it was about 8:00 a.m. My husband had pumped up the tires for me around 6 a.m. So the bike sat in transition for two hours and (I'm assuming) no one messed with it. I pulled over, got out my mini pump and filled it as much as I could (probably about half full because I was really flustered at this point.) Rode the 12.5 miles on basically a flat. After the race, my husband filled the tire again to see if it would lose air on the way home. It didn't. Tire is perfectly fine. Hoping this doesn't happen again and that someone more experienced with bike maintenance can help me!