Answer:
There is an accident ahead, which often causes traffic to slow.
Step-by-step explanation:
Looking at the question closely, we understand that the traffic jam occurred at a time that is not generally regarded as a rush hour.
A traffic jam due to road repair is not really an option because roads are closed during repairs. Shop malls announce their special sales days ahead of time so that many people will hear about it.
However, it is likely that an accident occurred. An road accident may suddenly occur on an otherwise less busy road. This often leads to an unusual delay in traffic irrespective of the hour of the day when it occurs.