The abduction happened right in the middle of the hysteria surrounding "satanic ritual abuse" and "recovered memories". I'm going to guess that, having heard about some of these cases, the grandparents questioned the children about whether they had been abused. Unfortunately, the reason that "recovered memories" have been mostly debunked is because if you are not *extremely* careful about how you go about this, you end up implanting false memories, which is what happened in the "satanic ritual abuse" cases. So, in trying to protect the children, the grandparents probably ended up just ruining the childrens' memories of their parents and led them into lives where they believed they were victims, and damaged, when probably absolutely nothing happened. The studies that showed the problem with recovered memories did not come out until the mid-90s, so it was long after the fact.
I would not suspect that the grandparents were doing this to take the children away, but rather in their desire to protect their grandchildren they made a really awful mistake and noone can convince them that the alleged abuse never happened. So sad.