Not to say that LR's don't have ground issues, ( and that is in fact the culprit for a myriad of problems on them)but that is not why all the lights flash, and if that fixed yours it likely means you just provided a better voltage drain due to some other voltage leak of which a Rover harness has plenty.
The stock wiring relies on the opposite indicator (signal) filaments to provide the ground for the flashers (the bulbs on the right side ground the left, and vice versa.) LEDs don't provide as much resistance as incandescents, so enough voltage gets through the dash indicator bulb to light the opposite side indicators.
This will happen on nearly every motorcycle you put LEDs on and it is the exact same principle. Single dash indicator allows voltage to flow both directions, thus lighting all LEDs due to the small amount of power required to operate them.