Some Caterpillars can defoliate entire trees.
Trees are a lot like people.
Give them tender loving care and, more often than not, they'll live to a ripe old age. But neglect good health practices, and they'll struggle to survive or, worse, keel over and die. A lot of times, people water too much or a tree is planted in compacted soil. Sometimes it's just a matter of really bad pruning. Any and all of these things can put a tree under stress and leave it vulnerable to pests and disease.