My iPhone called 911 by itself

You mean all these (given below) happened automatically?

Call emergency services

Here's how to make the call on iPhone 8 or later: 

1. Press and hold the side button and one of the Volume buttons until the Emergency SOS slider appears. 
2. Drag the Emergency SOS slider to call emergency services. If you continue to hold down the side button and Volume button, instead of dragging the slider, a countdown begins and an alert sounds. If you hold down the buttons until the countdown ends, your iPhone automatically calls emergency services.

Smart Phones neither have the intelligence nor has the hostility. Hence we need to look at our device settings and the way we synchronise our smart phones with Mac/PC/iCloud and also how careful are we in sharing our sensitive and valuable informations with friends and family members.