Pick 6's, or TAINTs as Bill Simmons refers to them, are a product of where on the field an INT occurse. A throw from the 5 yard line that gets picked off in the endzone has a good chance of getting run back 100 yards for a TD. Same goes for an INT inside your own 15 yard line or so.
Then there are the INTs that result when the DB jumps a shallow out or a curl pattern. INTs in the middle of the field (in both senses: between the hashes and between the 30s) don't generally get run back for TDs.
Interesting! I would have thought the closer you were to your own end zone the more likely
If you look at last season I think a lot of it had to do with the playcalling. All the plays were underneath and checkdowns. Picks on those types of plays are much more likely to lead to a TD than deep shots down the field where receivers and backs have chances at tackling the defensive player who comes up with the pick.