What does the Brown in Doc Brown mean?
serious shit! 
Why did Doc Brown get fired from the Taxi company?
Because TAXI is non-canon in the BTTF universe!
Do you have a funnier answer to that joke, perhaps having him outfit a cab with a flux capacitor and Danny Devito getting mad?
uhhh, maybe later
Why does Doc Brown keep going back in time.
To fuck with Wikipedians