The boots may get torn due to fol possible reasons..
Substandard Boots
Targetting the boots with high pressure water during service
Something touching and tearing the boots during its normal working and rotational function like a wire or clip etc
Over tightening the clamps n clips on the sides of the boots
misaligned CV joint and boots
Driving the car frequently on surfaces having branches or loose ends striking the boots and damaging them
Overheating wheels or engine
Somebody might like to add something..