Why would a wild magic Aasimar's memories be altered?

DISCLAIMER
First I would warn you about making backstories FOR your players without their permission. It sounds like your player might want that from you, but still tread carefully. And always be willing to change your plans and backstory to benefit the player's experience. It is after all their character and you already have a world to play with. Just be accommodating and make sure you don't pave over what your player wants to do with their character.
DISCLAIMER

But now lets get to the fun part.

1. I kinda like the idea of the beholder bit, but you are right in it possible being to much. HOWEVER what you could do is instead of having no MEMORY he could have no PAST. Maybe make it a possibility or a fact that Aasimar are born fully grown. They kind of just pop into existence and have little to no inclination to learn of their non-existent past (Which could make their character an unusual case if they do want to know)

2. You could also have it that his memory was taken by one or both of his parents because having him was kinda a whoopsy for some reason. Forbidden love, raising a child a hassle, child could be used against them, ect.

3. In the more mundane, the accident could have actually been an attack or theft. The guilty parties, needing a scapegoat to cover their crime, used a type of magic or poison that corroded his memories of the incident. He would wake up later and assume he had been the one responsible.

On a side note, I think I love the mumblemumble Guild. xD