I suspect in my case someone knew it was a camera backpack, knew it might have something worth money in it, and took it. We had other bags with us that were not bothered at all.
I think thieves have gotten smart - they know that most bags in strollers will be full of junk (snacks, change of clothes, diapers, sippy cups, etc) and they target things that look like they hold items that are worth money. I could kick myself as I know better, but we were at the Merida Meet & Greet when it happened, just steps away from our stroller, and there all of 8 minutes. Long enough to get something stolen, however. It still es me off and makes me sad an violated. When you're in you're happy Disney bubble it makes it even worse.