As children of course, our boundaries are handed to us by our parents, teachers, and guardians but when we begin to develop our own sense of identity and begin to interact with people who perhaps have different ideas of what boundaries are, we quickly find ourselves in conflict. It's really uncomfortable when we find ourselves in conflict, and resolutions can be difficult. Life isn't always fair, no matter how much we want it to be.
Somewhere in the mind of everyone is a niggle or a nag that needs resolution. If it's small, we can often work around it, but little things when left get bigger. A some point, we have to find a resolution. But how? We all can struggle with this, it's tender ground for most people. Getting a balance between consideration and assertion is paramount. Maybe when we're young, we do simply look for ways of dealing with our own boundary issues by replicating the situation somehow and gauging how other people respond.
A friend of mine made an excellent contribution to the idea, when they mentioned that many people with mental health conditions such as anxiety or PTSD not only do this but also crave attention. Usually when someone with mental health problems habitually breaks boundaries with people or fails to understand/care when they're hurting someone, it's termed a personality disorder. The spectrum of personality disorders can be diagnosed in varying degrees of severity and aspects of social life. The craving of attention is perhaps separate to this, and can be expressed in differing degrees to other symptoms. This is usually diagnosed as ADD.
Having thought about this, the mental health conditions mentioned such as anxiety and PTSD that can cause issues with upsetting other people's boundaries are usually separate but not necessarily unrelated phenomena. Breaking boundaries habitually or craving love and affection are not direct symptoms to either condition however can be related due to the manner in which the condition was acquired. A soldier from a war suffering with PTSD may not necessarily crave validation from anyone and may only break societal boundaries when in crisis, which in medical emergency terms cannot be considered usual behaviour. Anxiety is similar, it is having a strong sense of fear that means we have anxiety. The basis of this fear is usually pre-learned through continual exposure to frightening situations. Again, this does not necessarily lead to breaking people's boundaries or craving attention.
What may be the case is that in the manner of childhood trauma, the lessons learned on a foundational level make it difficult to mature past particular thinking. We get stuck in adolescence in one or more facets of our mental lives because of serious blockages in our learned thinking. Unlearning something deeply embedded in our subconscious takes many years of purposeful re-thinking. It's extremely difficult to achieve even small progress. This is why child abuse is so insidious in nature.
So the reasons behind why young people push other people's buttons are probably quite varied, but the main reason is that they are young. Maybe we need to lighten up a bit as adults, and remember that in psychological terms, we're much bigger than they are even when they are as physically tall as us.