Pets Mate On Their Own
I have experienced a bug many times where pets will breed on their own. If I have an opposite-sex pair in the same household and ever forget to spay/neuter them, they always seem to breed and I get an "expecting puppies/kittens" message without ever prompting anything.
Also, this issue has happened even when one pet is fixed and the other is not. I had one pet neutered to prevent a pregnancy from happening, but since the other dog was not fixed, they still somehow mated and had a litter. The only way I have been able to avoid pet pregnancies is to have all pets in the household fixed.
This bug has been going on for a while, I have had it happen at least 10 times across multiple saves. I do use mods, but they are up to date and appear to be working.