It would appear that the 555 is free-running so when it goes high is random as compared to when a button is pressed.
It also appears that K1 will not do anything when the brake is applied since it is applying 12V through the turn-signal switch that is already continuously high from the brake signal.
I assume you want the tail-light to blink when both the brake and turn signal are active.
That would require some type of inhibit/logic function to block the brake signal when the turn signal is active.