Yes, you have no way of knowing. But that's why they are letting you know when, or as soon as, it becomes an issue for them.

I don't think it's 'at some random time'. Sometimes people can be fine with something but as time goes on or something changes for them, they realise they are no longer fine with it. Sometimes people can also change their minds.. That is the nature of human beings. Something can't be 'fine' forever... or 'not fine' forever. There is fluctuation for whatever reason.

It seems this is what happened with your T. He was initially fine but over time it was less so. I believe you asked your T to tell you when this happened. So, he told you at the first (appropriate) opportunity. He did stick to the 'deal' you both made for him to speak up when/if something changes for him. He honoured that deal and didn't stay quiet and let it fester or turn into resentment.

But now it seems you feel bad that he told you... Just as you had no way of knowing, he also had no way of knowing he would feel differently down the line.