It's called fast flash and is mandated by FMVSS (Fed Motor Vehicle Satefy Standards) to indicate to the operator that a bulb is burnt out. Could be intermitent, could be front, rear, side flashers. Just depends on the flasher system. Look at the side that does not do this and not which bulbs flash. When it does happen on the other note, leave it going and then check out which one(s) are not flashing. That's the one needing replacement. Care on replacing the bulb with the same on as some manufacturers use resistance(current draw) to determine outage conditions. Just my 2 cents.