Overexplaining rarely feels like a boundary problem. It feels like being thorough, or fair, or considerate. It feels like making sure the other person understands.
But underneath the explanation there is often a quieter belief: that a decision does not count until someone else agrees it was reasonable.
Where it comes from
In environments where saying no had consequences, explanation became the negotiation. A long enough justification might prevent anger, withdrawal, or the loss of connection. It worked often enough to become a habit.
A boundary that requires permission is a request.
This is not an argument for being abrupt. Context is a normal part of close relationships, and generosity in communication is not the same thing as self-abandonment.
A place to start
Notice the difference between offering an explanation and needing one to be accepted. The first is a choice. The second is a cost.
Practising the shorter answer is uncomfortable at first. It usually becomes less so, and the relationships that can hold it tend to be the ones worth keeping.