Yorkie humping after…….?

I fostered a 9 month old yorkie puppy (had papers) from the local yorkie rescue here in my town for 4 months. His owners could no longer keep him, so they surrendered him to the recue and he was given to me to keep during the time that that rescue could find his permanent home. A few weeks ago, the rescue called and said that Jack (the yorkie) had been adopted and I was to have him there the next day. So, I did, sad to see him go, but since I had known he was going to be adopted, fine with it and glad he now had a family. (And, of course, I gave them back the AKC papers, vet records, etc.)
Now, here's where the problem started.
A week after returning him to the rescue, I received another call from them. Apparently, Jack's new owners had had TONS of problems with him. Biting them when they tried to pet him, tearing stuff up, etc. None of this was his behavior when he was with me. The owners said they were returning the dog to the rescue. They asked if I would foster him again until a new home was found. I said yes, and honestly at this point, figured that I might just have fostered a dog that was going to have severe seperation anxiety from my home, and that he may never be able to go somewhere else, sort of that Jack had decided for himself that WE were his permanent family, and started dropping the hints to my husband that we probably had another family member….
But here's the thing. When Jack came back to my house, he was as good as he always was, which could be explained by the separation thing, BUT he now HUMPS everything!!! Pillows, my other dog, my leg, etc…. He NEVER did this before.
My concern is this. I wonder if the people who took Jack into their "family" bred him. If maybe they used a pretense that they wanted to "rescue" the dog, but had just figured out a free way to get stud service. And yes, it was a part of the contract that they signed that they would have to have him neutered, but of course they were given the time to set up an appointment, etc.. So a mating could have occured in those few days he was gone.
And frankly, I am p****d!
I have never bred dogs before, so does anyone know if a dog who normally doesn't hump, will begin doing so after a first "experience" as well as after just being taken away from an estrus female? Is this behavior a sign of a recent breeding.
ALso, they did return the AKC papers, but could they have just taken Jack's info (registration number, etc.) from his papers and then be able to register a litter of puppies and then sell them AKC registered without producing the actual document of the father? And is there a way to check, a few months from now, if there is a litter of puppies registered with him as the sire?
And any advice on what to do about this regarding the rescue or temporary "owners" would be appreciated.
I realize that the humping will stop eventually, and certainly if I keep him, he will be neutered. (Same as my female yorkie who is permanently mine is spayed.) It is not the humping that bothers me, I know it will stop, just looking for opinions on what this behavior means, as well as the shadiness of people.
And he's no longer 9 months now. That was how old when I started fostering him. He's now 13 months. Thanks!

It could be that Jack is feeling insecure, and trying to re-establish his place in your home. Humping things (especially legs, other dogs, etc.) is more about dominance than sex. The only time it's about sex is when there is a b*tch in season in the area.

I suppose it's possible that these people could have done something like this — I'm surprised that the rescue turned over his AKC papers. Most rescues destroy them simply to avoid any issues like the one you're concerned about (of course, most rescues spay/neuter animals *before* placing them, as well). However, assuming that they did breed him and copy his reg. number, they would still have to have the address and signature of his owner (as listed on the registration papers) on the litter registration.

I suppose they could simply forge it, if they were dishonest enough to use him in the first place.

I am never one to underestimate the sliminess of human beings, but I do think maybe you're over-thinking this, and Jack is just insecure. I'm sure he'll get over it.

I've seen people come up with all sorts of "bad behavior" excuses in order to return an otherwise angelic dog, just to avoid admitting that they can't deal with the responsibility. So they probably were losers. But that doesn't necessarily mean they bred him.