I’m sure this is just a random glitch that only happens so often, but I thought I would tell y’all. I was descending into Barcelona and all of a sudden I start falling out of the sky. I didn’t touch anything and I had just turned on VNAV. I was flying the PUMA1H arrival if that means anything. I’m also aware AP disengages when you push the vertical speed limits which is the only place I’d seen this before, but wouldn’t VNAV not do that and again I didn’t touch anything and it normally regulates itself. So, again I’m sure this was some random glitch, but just in case it isn’t thought y’all should know.