That's the Live Photo feature. It captures the action 1.5 seconds before and after you press the shutter, including audio!
Yes, even on still pictures!
I guess you still have the iPhone 6, as Live Photo is on iPhone 6+ and later.
It's a feature that is on by default. If you don't want that feature, you can turn it off by pressing the tri-circle icon on the camera app screen.
When you launch the camera app, find the Live Photo icon, which is made up of three concentric circles at the top of your screen. If the icon is yellow (or gold), Live Photos is currently turned on. To turn it off, simply tap on the icon. You can always turn it back on by following the same steps. It's one feature that's easy to quickly turn on or off.