If you have any other contactless cards in your wallet (like a credit or debit card) then the faregate will pick those up also. You have to take the SmarTrip out (or remove the other cards from your wallet) to get rid of the interference.

If that's not it, I'm curious if someone with knowledge will comment. But it would make sense to me that they'd tune down the transmit power on the faregates, with people carrying multiple phones and cards, you'll get more false signals the more power you give the radio signal. It's better for security and may also reduce confusion/congestion at the faregates if they lower the range down to a few inches. I don't know if that's it, but it would make sense to me if that's the explanation.