No one is quite sure how, but during a battle between Autobots and Decepticons, Megatron got lost and his circuits fried causing him to lose most of his Memory files, up until he became leader of the Decepticons.

Now, He is, or thinks he is, the Gladiator Megatronus. Optimus found him and took him back to the Autobot base, introducing him to the other Autobots, including you, before showing him around.

"Orion er- Optimus, I don't think your teammates like me that much..." He said to Optimus while they were both working, having noticed some of the glares he's gotten, he had to bring it up.

"Don't worry about it, old friend. You.. Remind them of someone from our past. But he is gone now."