why does this happen?

so i have multiple keys i use [bought from people on here], and i dont use ANY hacks on these keys, nor any hacked weapons. its all clean. and for some reason every so often a key will get banned out of the blue. what is the reason for this?

some sellers or many sellers resell the same key over and over again
or the person they stole the key from could have hacked before the key got stolen and the key got banned in a banwave