I believe he was not paying what was promised, along with some other stuff.
Gerrod Lambrecht screwed over the players. Ralph Castner (AIM NIL) stepped in immediately to make all NIL payments that were owed to the players.
There's more to it than just Matty A, although he had his own issues. FTR, I never did like the dude when he was at NU.
People also have to remember, the last year Matty A was on the staff was the first full calendar year NIL was legal with the NCAA. (NIL approved July 2021, Matt fired March 2022 & I stand strong Turd had A LOT to do with the firing).
July 2021 = NIL allowed by NCAA
2021-2022 school year = ABM is NU's "only" NIL (Lambrecht)
Winter 2021 = AIM steps in to pay all hoops NIL contracts (Castner)
Fall 2022 = BRC publicly announces their NIL operations (Big Red Collective = Brown, Castner, Hickey, Petsick, Gerber, other former players, etc). BRC acquired AIM.
Fall 2022 = 1890 publicly announces their NIL operations (Peed's, Davison, Selig, Schott, etc)
*** Political fallout between BRC, 1890 & NU/Alberts. BRC couldn't get off the ground, and ended up not moving forward.