Unauthorized Transactions and Reversal

[MrG]

Two years ago I had an incident where a transaction went off my credit card with "Bank A" and then immediately afterwards a reversal SMS came through. I didn't waste anytime and as I was driving at the time I immediately diverted to a Standard Bank office where they promptly cancelled the card and placed an order for a new card.

This evening the same thing happened again with "Bank B". My questions are:

What is the nature of these transactions?
How are they possible?
How does the bank know to reverse them within seconds without my input?

Does anybody have experience with this?

 

[Venomous]

often...

scammers got your details
they transact for a few rands,
then they reverse (test working card), merchant reversal.
then either the same day, or within days, they hit hard...