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…