According to Brian Costello of the New York Post, Joe McKnight was pulled over after passing a police officer at a high rate of speed and changing lanes without signaling during a Jets off-day on July 30.
When the officer ran the license plate, he discovered a pair of fines that had gone unpaid by McKnight. The first was for $200 for failing to produce an insurance card in Summit and the second was $120 for failing to observe a traffic signal in Springfield. He was subsequently arrested and paid the outstanding charges of $320 and was released with a summons for an unsafe lane change and uninsured motor vehicle.
Source: Curtis Crabtree, NBC Sports Pro Footabll Talk Jets RB Joe McKnight Arrested For Unpaid Fines by Blake Bass