It is from the online journal Psychology Today. Here it goes:
Some of those behaviors can become habitual — i.e., habits. The resulting chain — trigger activating motive, leading to behavior, leading to repetition, which eventually leads to habit, a specific style of responding — follows the social learning theory traced by Julian Rotter in the 1950s.
I cannot get what trigger activating motive is. If the word trigger was used there as a verb then why was that used in the infinitive form? Should not it be triggers activating motive? And Why was the definite article was omitted before the word motive? Could you please rephrase the whole sentence for me?