Most likely that the absence of movement data is perceived as sitting/standing. Since the stand goal requires standing and moving for at least one minute (the purpose of the stand goal is more to get you moving for at least a minute rather than just stand up - just standing up doesn't count), if you haven't moved for a while (50 minutes in an hour) then it gives you a nudge to do so.

It's a somewhat confusingly named metric. It actually measures "have you been moving around for at least one minute this hour?", where I think "moving around" is basically defined as generating steps.

My reason for this theory is that I sometimes bounce my leg when I'm sitting, and if I have my arm resting on said leg this sometimes triggers the stand goal for the hour.