There could be a number of reasons for this. Some are avoidable, others are not. I will use the route 133 from Liverpool Street Station to Tooting Broadway as an example, for reasons which should become obvious. Buses leave Liverpool Street every 8 minutes during the day (6 minutes during peak times) and are clocked out by the fat controller (actually, Roger is not at all fat but I never grew out of Thomas the Tank Engine).

Scenario One - Bus 01 leaves LS promptly at 9am and drives round to Moorgate. On arrival, by coincidence, a train has just arrived and 50 people get off and crowd onto the bus. This takes 6 minutes. Bus 02 leaves LS and finds the Moorgate stop empty as well as all subsequent stops. By the time he reaches Bank, he has caught up with Bus 01. Traffic is heavy and by the time both buses clear Bank, Bus 03 is leaving LS behind them. The first two buses reach London Bridge Station and more passengers board both buses to capacity. Bus 03 is already crossing the bridge behind them to find an empty bus stop. By the time they reach Elephant and Castle and both the first buses take time to unload, Bus 03 is right on their tail. They cannot avoid running in convoy through Kennington to Brixton.

Scenario Two - Every company (including London General who run the 133) has its lazy people who are allergic to work. Some of these have designed their own sequence to make sure that other drivers carry all the passengers. Bus 01 leaves Tooting Broadway at 5pm en route to Liverpool Street knowing that there will be a huge number of passengers coming back out of the City. Bus 02 should depart 6 minutes later (also knowing the forthcoming busy situation) but does not do so. He hangs back until he is late and then radios the aforementioned fat controller and tells him that he is running late. To keep most buses on time, the controller tells him to "turn" at London Bridge Station to bring him back onto time. Driver of Bus 02 is happy because it means that he will avoid carrying the main rush hour traffic out of LS and get more stand time to read his paper, etc. Bus 01 has to carry double the number of passengers to normal and this makes him late. Bus 02 does not leave London Bridge on time but waits for Bus 01 to go through so that he can follow him down the road empty, thus avoiding doing any work. The justification for this is an expression much used on the buses - "take it easy, man". When "take it easy" means "do not get upset and just do the job", this is ok. When it means "let the others do all the work", it is unacceptable to passengers and to other drivers. Oddly, management does not seem concerned enough to prevent this happening.