Here and here respectively. Both suggest “contempt of cop” may have played a role in the decision to arrest Gates.

Lowry:

The interaction spiraled downward until the officer arrested Gates, basically for the offense known as “contempt of cop.” In a tragicomic touch, the cuffed Gates needed his cane to get to the police car.
The officer, who has an exemplary record and has taught a police-academy class on racial profiling, probably should have shown more forbearance. But interactions with police officers typically end better when you don’t verbally abuse them.

Mac Donald:

It is certainly possible to debate whether Gates’s escalating verbal abuse of the investigating officer and refusal to cooperate with his requests rose to the level of criminal conduct. Most certainly, it lay within Sgt. James Crowley’s discretion not to make the arrest — and in retrospect, it would have been preferable if he had thanked Gates for his cooperation and walked away from the provocation. I would guess that Sergeant Crowley simply snapped under Gates’s taunts and chose to teach him a lesson for the informal offense of contempt of cop — an understandable, if less than ideal, reaction, but not a racist one. Crowley, even by Gates’s account, acted politely throughout the interaction.