I'd have a talk with them and make it unmistakably clear that, unless they want to grow old(er) without ever speaking to their grandchild again, they're not to pull this shit again. Ever. As in there won't be a second warning. There won't be an 'oh in that case...', either. Do this again and I'm cutting you out of my life.

As for the reasoning: Love and concern is one thing. This isn't about love and concern. Your mother knew where you were. Your mother - which I'm assuming to be a normal, i.e not insane person - okayed your stay at your bf's. You're her responsibility, so everything's fine. There is zero reason for your grandparents to get involved - let alone to just involve themselves. There is zero reason to assume that you were in any sort of danger. As such, them still doing this to you has very little to do with love and concern and quite a lot with the old teaching-you-a-lesson kind of approach to raising a child.

I'm not saying to actually cut off contact, ofc, but a threat like that usually gets through. Having grown up with a father who is, in some ways, not unlike your grandparents, I've yet to find a better way to handle these situations. Some people are simply unable to graps the concept of personal boundaries, unless the price of overstepping them is so great that they'd rather not try.