You can be cut out of your AC life without having done anything to deserve it, it happened to me. I loved and adored my DD, she lived with me with my GD, we had a very happy & supportive life, I wasn't just a grandparent but the second parent in my GD life, as the father was never in her life, nor any of his family.
The reason cut out GP's long to be reunited with their GC is that although missing the AC as much, as I do, if reunited, the AC will be almost the same person, save a few years of aging, were as the GC will be completely different, won't know who you are from Adam and the close bond you had with them gone. My GC were 2.5yrs & 18mths, now 8.5yrs and 7.5yrs, big, big difference in the child, quite grown up from being the toddler you knew and loved and you just a stranger to them 