According to mccardell, what sparked the fight between bonus army veterans and the soldiers?

The answer is Peaceful protest

The bonus army veteran consist of the veteran from world war I that conducted a protest and asked for cash-payment redemption for their services in the war.

Since this protest happened during the great depression, things like money became a really sensitive issues even among other currently serving soldiers (becasuse it most likely to be taken from their bonus fund) and eventually the fight happened.