Certified GKH Gardening Expert
There are many things that could be causing this.
If the side that is dropping is facing a structure, then it may be too little light.
If the side that is dropping is facing out (while the healthy side faces a structure) it may be wind or winter burn, where the wind or cold dries the leaves out causing them to fall. The side facing inward would be protected.
It may be a fungus. Typically, you would see black or yellow spots on the leaves if it was a fungus. A fungicide will help clear it up.