It's usually a butt call, it easily happens in your pocket or bag, specially if you have other items in it as well. Some phone/cupholders in car might be just the right shape to press the power button repeatedly when turning or going over bumps.
Sometimes I wonder how can people not notice they're calling emergency, because the phone usually makes loud noise and counts down before actually calling... But then I realise that if I have switched off notifications on my smart watch, I often miss a call ringing in my pocket, so...
If I can't have control over shit happening, then I just don't care.
I think it was Další Lama who said: "If a problem has a solution, there is no reason to worry. If there is no solution, worrying will not help."