My Sportster front signals are always-on running lights.
If yours are brightening upon actuating the brake, you may have juice flowing back through your brake's 1157 bulb from the brake light circuit to the running light circuit.
Pull the bulb out of the brakes and take a peek at it and the housing contacts.
Might be though, just a bit of a surge upon the demand for the brake light too and be nothing to worry about.
Todd.