The reason does not matter (especially since we are unlikely to hear it from anyone), what matters is that they refuse to say what it is. "Effort" is not an excuse either since they even replied to him - but with something unprofessional while still leaving out the reason for no good reason that I can think of. You can even mask it behind a more fuzzy reason if you think the specific reason is not good enough to be made public, but they did not even make the attempt.
> then muting communications is the least offensive option
Only after telling him. Something, anything - they could even make up a polite "official" reason and keep the truth for themselves as we humans often do, which can actually be perfectly okay. But stubbornly refusing to give any reason at all... no, that's not okay. It would be if it was between individuals, but not for a professional organization, especially one that the person may not be able to ignore even if they wanted to, since he seems to find them a valuable contribution for his professional life.