Why does my iPhone make calls to my emergency contacts on its own?
Following hints from the Reddit post:


  If your iPhone is in your pocket and the screen is facing your body,
  the motion that your legs make when walking up a set of stairs "taps"
  the bottom left corner of the screen (at least for me).
  
  the first tap registers as a swipe up, which, because FaceID is not
  recognizing a face (because it's in your pocket), goes to the passcode
  entry screen
  
  2) the second tap clicks the Emergency text in the bottom left of the
  unlock screen
  
  3) the third tap clicks the Medical ID text in the bottom left of the
  Emergency screen
  
  4) the fourth tap clicks the phone number of the second emergency
  contact at the very bottom


... I found a solution that at in most cases prevents accidentally calling my emergency contacts:


Create a new contact called "Dummy" or similar, with an invalid phone number (e.g. 12345678). Make sure you have at least one other emergency contact.
Go to the Settings app &rarr; Health &rarr; Emergency info, and find the Emergency contacts list 
Reorder your emergency contacts so that "Dummy" is number 2 from the top


This setup will cause your Dummy contact, and not a real contact, to be called when the bottom part of your screen is tapped in your pocket.