Camera Opens Accidentally!

How can I prevent this from happening?

I tend to carry my iPhone either in my back left pocket, with the screen facing my body or in my shirt chest pocket, with the screen facing my body. Several times, in both carrying positions, I have extracted my phone to discover that the camera app is already open! In one instance, when I had the phone in my shirt pocket as I was riding my horse, I also discovered that the camera had taken over a dozen pictures before I retrieved it from my pocket!

This reduces the battery life, as you would expect. When horse riding on trails I use a GPS app, so I am very conscious of battery life. And, I take a lot of pictures and videos to share later. Having the camera app activate on its own is not only inconvenient, it is battery wasteful. Since noticing this, I have taken extra precautions to make sure I turn the phone off before storing it back in my pocket. This is really baffling!