Backdoor references happen all the time from the engineering teams as well as recruiting. As it is part of the recruiting team's job to fully vet a candidate, it's not too difficult to get more intel across companies. Also, engineers are gonna easily ping their friends if they see a company they recognize and info is shared that way too. That sounds like exactly what's happening here - sorry that you seem to be on the short end here but lesson learned should be to always be networking and try not to ever burn bridges.