It's hard to even believe people would consider something so ridiculous, but the theory that Tarzan's parents are the same as Elsa and Anna's got enough traction to earn the attention of Chanel Frederator's "Cartoon Conspiracy" segment. At the same time, Emily (the host) clearly understands the implausibility of this theory.
But where does all of this nonsense come from? As it turns out, someone pretty close to the source — the director of both Tarzan and Frozen, Chris Buck. Disneytheory.com breaks it down quite nicely, but it basically boils down to Buck being a bit of a troll. He made the connection in a reddit thread and the Internet went with it.
I will go on the record saying that this cannot possibly be true, but I'd be curious to see someone try and defend it. Anyone up for a challenge?