Chosen Solution
Phone opens apps when locked. I particularly know this when it opens YouTube because I hear audio. However, it will open the calculator and other apps with no audio. This is happened since before the last update. I thought I solved the problem when I deleted Pandora when Pandora used to spontaneously play music. It did not solve the problem. My AT&T store checked my apps and found an app that was not working properly. I deleted it, but the problem prevails.
I recommend going for apple warranty. It’s probably going to be covered if it isn’t caused by physical damage to phone like drop.
