Fall Detection.
A fall is detected by the gyroscope and accelerometer and can give false readings if you're a particulary active or younger person which is why it's recommended for people over 65,it's usually triggered by a short sharp knock with your watch wrist,to compare it's like banging your clenched fist on a table,the movement of your wrist is detected then it comes to a sudden stop and this usually triggers the fall detection.
One reason the watch may not detect a fall is if your watch wrist is somehow cushioned from the fall and the watch doesn't detect that short sharp knock.
Another reason could be that the watch is worn a little bit too loose and the wrist movements aren't being detected properly.