Tricky! The word "to" is simply acting as a preposition here. It is not forming the infinitive form of the verb.
"To" is the natural preposition following "approach." "X is a method for Y," but "X is an approach to Y."
"Understanding" is functioning as a noun. Mentally, it is the place to which the approaches lead.
Here is a series of sentences, see if yours makes sense after you read the others.
"On this highway, the approaches to the George Washington Bridge are confusing."
"Behaviorism is one approach to psychology."
"Behaviorism is one approach to understanding human behavior."
"Psychologists have adopted several appoaches to understanding aggressive behavior in people."