Firstly, gosh! pitfalls of grandparent babysitters is a WHOLE thread in itself!
There is SOOO many variables to the back story, that just to say "cut off over babysitting" is too much of a tip of an ice berg to derive a right and wrong from
I have friends who work full time and childmind their GCs on their days off and I think, my god! they have NO time off yet their AC are getting all this "me time" while the baby is at nannies!! But my friends say they're happy to do it and wouldn't have it any other way so I don't say anything, but inside I'm thinking that the ACs in those cases are taking the piss with all the babysitting.. maybe I shouldn't judge, but we all do a little don't we?
On the other hand, I have parent friends whose Mother/MIL was upset that they were considering nursery/childminder and insisted they were happy to do the childcare, only to repeatedly cancel at short notice leaving my friends in both a difficult position with work, but also accused of taking the mother/MIL for granted by being upset about last minute avoidable cancellations.. you can see the other side of the narrative "oh! she's in a huff with my because I didn't babysit last week".. when the parents side is the GP agreed to it, then backed out when it was too late to make other arrangements.
Other friends are bullied into letting their babies go for sleepovers at nannies house when they're still very young and the mother isn't really ready for the baby being away over night.. and yet they are expected to be endebted to the bullying mother/MIL for giving them "a break" that they neither wanted or needed or asked for.
And yes, there are mothers who get in a huff if they can't go socialising every weekend with free babysitters. Try posting as one on MN and see how you get on though 
Also there may be AC who don't want a relationship with their parents, but who don't see them as a danger to their own children, so might decide that the best thing is for the children to go to the grandparent's house without them so that the children's relationship with the GPs isn't affected by the tensions between GPs and ACs. The "other side" narrative to this might be "we never hear from her unless she wants something from us like babysitting"
And there'll be a thousand other pitfalls that could result in the nut-shell version of events being "they NC because I didn't babysit"
Usually it's more a case of straws that break a camels back I think.
I've been following the thread on MN too, and want to write "so what will your children say about you? A woman full of hate, and desire for revenge?"
If I was out for revenge I'ld be in contact with the person I wanted that from. I don't want anythning from her, that's the point.
I'm not posting identifyable details on blogs or newspapers for the kids to read, I'm keeping them out of it and letting it out in appropriate settings such as anonomous discussion forums about the topic. They don't need to see any of the nonsense, that's the point of going NC. They don't need to hear what I think of what's gone on. I'm not interested in using them as soundboards for "my side of the story" or anything like that. If I was, I might as well re-introduce them to the car-crash that is being in a relationship with their grandmother!