From iA wiki
Limited bandwidth capacity or slowing of distribution, including lag, defragmented hard drive, interference from noise, old hardware, traffic jams and the Slashdot Effect. Using past historical data and extrapolating trends, future bottlenecks can be predicted with a relatively high degree of probability.
Bottlenecks may arise from various social situations. Studies of human bottlenecks indicate people tend to panic and behave in a manner that exacerbates the blocking effect. For instance, with hindsight people escaping a fire in a room who all slowly walk to a single door, instead of falling over each other in a stampede, are more likely to survive.