Stress/anxiety/environment are the biggest, but some level of genetics probably plays into the severity as well. I find that my stutter is at its worst when I’m particularly stressed/anxious, or I am putting pressure on myself to speak quickly and/or without stuttering. It’s difficult because sometimes it’s hard to gauge and thoroughly pick apart how much stress your body and mind are under (and where it’s coming from) so it can seem totally random, not to totally write off the possibility of it being random sometimes.