It's most likely the "fuk" in it. They've probably got some sort of substring logic and noticed the "fuk" and concluded that you've emulated a profanity in your moniker. Perhaps the "u" as well. Put them together, and what do we have? Hmm...
"Yay! We did it!"
"Who are you?"
"Um, uh... just ... a guy." *flee*
"Who are you?"
"Um, uh... just ... a guy." *flee*