These are my favorite.

When I was in middle school I chose the name CrazyInsaneManiac as my chat name. People did not like to type that out for very long and since everyone got called Crazy at some point, my name was shorted to CIM by a lot of people. I started going as CIM, so people started stylizing it as Cim, so I started going as that and then people started calling me Cimanim to be cute and it stuck, but then people started calling me other names that were longer than that like Cimberly and CimminyCricket.

And that's how I got my nickname. People did not like how long my name was, so they shortened it and then made it long again.