Road rage: Someone is unhappy with traffic enforcement
From the Evening Standard: Three injured in new letter-bomb attack. This is the fourth in a wave of letter-bomb attacks, one attacked Capita, which administers the London Congestion Charge, one Speed Check Services, which does traffic enforcement, and one today at DVLA, the Driver and Vehicle Licensing Administration.
It recalls the The Mad Bomber of New York City in the 1940s, though the pace is much faster in this case.