The LED lights cause the Flasher, if it is an Electro-Mechanical flasher, to flash fast. This is a design of all flashers, including fully electronic ones so that the driver has an indication of a light bulb failing.
The problem with the electro-mechanical ones is that they measure the NET voltage drop in the circut, and if it rises above a certain setpoint the flasher behaves via a rapid flash.
Since a LED lamp has less voltage drop than a Incandecent lamp (LED lamps are current driven, and often have their own current source that limits their voltage drop) the electro-mechanical flasher thinks the light is burnt out and goes into it's rapid flash mode.
Adding a resistor to the line going to the lights adds a voltage drop that then avoids the fast flash. This also disables the circuits ability to detect if the lamp burns out.
Changing to an electronic flasher eleminates the problem as it uses an IC to measure current flow rather than voltage sensing, so it can tell the draw that occurs when a lamp is burned out vs when all lamps are running.
Thus with an electronic flasher and LED lights, you will still get the sensing ability for a failed lamp.
Electronic flashers are fairly new to be generally available, thus most people still sell in line resistors to deal with the fast flashing on cars.