Because People Want to!
It is not a Glitch or Anything,so don't worry.
For Some Odd Reason People like to Name Pokemon after Legendaries. I Find No Reason to nickname a Pokemon after Legendaries.It Doesn't Make them Cooler Either. A Charizard named Arceus won't make even the slightest difference. People who trade it will get stuck with a Pokemon with a Horrible Nickname.
Sooooo, they Nickname them that because they want to,not a Glitch like I mentioned.
Hope I Helped-Legend Of Lotad