In a freeroll you can't give people credit for skill until the later stages, and even then you have to wonder. You call there simply because you don't know his range and pushing with middle pockets is an entirely reasonable move. Don't get so worried about being tricked that you forget the strength of your own hand (which is what led you to raise in the first place). You are getting good solid pot odds, and you cover.

Edit:Bigstacked I limp AA, KK AK and even down to JJ a lot from early, the only time I'd ever raise was if I was sure some shorty was ready to reraise, or if my table was populated with experienced, opportunistic reraiser. Then I'd be looking to catch one of them. I consider 10k at 200 blinds to be bigstacked.