I think the answer to your question is rhetorical - YES it is a rebound, and why do some do this?
They feel the need to socialize in a religiously organized setting, it's as simple as that. Good people still feel the need to "belong" somewhere, it fills a spot in their lives,
Not everyone has that feeling to do that AFTER they leave the Borg, some do, so let them do it if it makes them feel better and fills a gap - and here is the key....> "IN THEIR LIFE"
If that isnt' for you we understand, but to ask that question is INSANE, It's like saying when you break up with your girlfriend and next week start dating another women, is that just a "Rebound" YES YES YES
Who gives a Rats Monkeys, what the universe holds or the answers to all the Galactic questions, WE'LL NEVER have the answers to that. Heck, Einstein, Newton and Stephen Hawking do not even know the answers, why do you think WE SHOULD HAVE THE ANSWERS? There may be no answers at all, and what a folly that is looking for something that does not exist.
Like Jack Nicholson said in a FEW GOOD MEN - "we can't handle the truth" that is why we live on earth for up to 80-90 years, if we are lucky and dont get stuck in a sad Nursing home, and DIE . YES WE DIE, Worm Food, 6 feet under, Dust to Dust.
That's all it is, why waste your time, with wondering why, we'll never get the answers and all we do is lose valuable time for the more enjoyable things: Playing with our grandkids, or our best friend "Sparky" , having coffee with Mary at Cafe Nervosa or traveling to the Andes to see the ancient world of people that just like us - never had the answers either.
Just get out there and do something good...er great with your life!
Thats my take, I have to go bed, I got all worked up over this, sorry folks