I once went to a seminar put on by a lighting manufacturer. The only thing that I remember about the context is the fact that if a HPS fixture cycles on and off, it is the bulb. Through the years, I have followed this rule, and have found it to always be true.
Some years back a shopping center had the same thing happening to an entire parking lot. They later found the cause when the circuit breaker caught fire. (It was arcing internally until it finally failed.)
So I would think that anything which causes an arc fault on the circuit would cause that effect. (Bad underground splice/feeder, etc.)
Arc faults would chop the current so fast that the bulb wouldn't go completely out, so it can strobe faster than the igniter would pulse it.
Were you a good samaritan and called the highway dept.?