It does not matter if a boundary makes sense to you. It does not matter if it seems inconsequential to you. Boundaries are the prerogative of the person who sets them. You do not know that person’s story, and they are not obligated to justify their boundaries to you. That touch that seems insignificant to you may be uncomfortably intimate for someone else. That interaction that is fine with others may trigger someone’s PTSD. You do not know more about someone than they know about themselves. Trust that they know what they are doing when they set a boundary with you, even if you do not understand why.
When someone sets a boundary with you they are saying “no.” No means no. Do not push people on their boundaries or ask for explanations that are not readily given. Doing these things indicates that you do not respect their boundaries. For many people, saying “no” once, setting a boundary, is difficult enough. Do not put them in a position where they must repeatedly do so. No means no the first time. Pushing them on it suggests a hope that you can wear them down, which is problematic at best and predatory at worst. No means no.