Maybe some were weak as active Witnesses because they didn't fully accept all of it. It's hard to get behind something when you sense problems with it, even if you can't fully explain them.
Then, when they fully woke up, they became zealous against it because they can fully see the trickery Watchtower uses and are happy to explain it to others.
Am I right in thinking that the most zealous JW make the most zealous apostates?
I think it's a general rule with plenty of room for exceptions.