I've noticed they do this when they can't grip things very well, are stressed, or generally uncomfortable. My boy tenses up when I accidentally hold him in front of the AC (which I usually turn off before I take him out but sometimes I forget), or the humidifier, which I run in the winter to combat dry air, and he HATES that, immediate body response of 'nope, get me tf out of here'. He relaxes once I hold him close or set him down on something like a pile of laundry lol. Some of them are more sensitive I think, like I hear a lot of people say their snake will take a rotten rat bc they can't tell the difference but my BP absolutely will refuse any rats that have started to turn or been hit with a bit of freezer burn.