#1 - They are self-centered and/or narcissists. It's all about them. They enjoy attention. Other people and their opinions or feelings are less important.
#2 - They have poor or no manners. They believe manners don't apply online, or apply differently. Or they simply don't know any better, having never learned decent manners in the first place.
#3 - They enjoy creating conflict. They don't know how to relate to someone else respectfully or cooperatively.
#4 - They are emotionally underdeveloped, i.e., juvenile. They enjoy making fun of someone else.
#5 - They are substance abusers. Or they may simply be intoxicated at the time.
#6 - They are angry, jealous, unhappy, and/or empty. The only way they know how to deal with their own feelings is to spread their own misery. They don't feel good about themselves, so they try to make someone else feel bad also.
#7 - They are bored.
#8 - They are lazy, especially intellectually lazy.
#9 - They don't have a firm grip on reality. For them, an online person is something less than someone in their physical presence.
#10 - They are anonymous, or nearly so. There is seldom any consequence for bad behavior.
#11 - There is no peer pressure to behave well. In fact, there is often peer reinforcement to behave badly, and they want to impress someone else.
#12 - They feel slighted and want to "get back" at that person.
#13 - They have a need to exert control. They can do so by being disruptive.
#14 - They are socially maladroit. They don't realize they are behaving trollishly in a particular setting.
#15 - They have no moral compass. Trolling doesn't seem wrong to them.
I can't think of anymore right now. Did I miss any? Feel free to add you own.