There are mainly 2 reasons that cause a “Bad Tick”.
1. Accidental “Bad Tick”
It is possible that your broker or one of the liquidity providers accidentally send false price feed.
In this case, the “Bad Tick” will be considered as a system error, your broker will try to restore the positions closed by the event as such an extreme price caused by “Bad Tick” doesn’t even exist at all.
2. Intentional “Bad Tick”
Some market makers can cause a “Bad Tick” intentionally.
In this case, the event is often called as a “Stop hunting” and the broker may refuses to restore any false numbers came out by the event.