Bus riders in San Francisco are getting their kicks by kicking open the rear doors of Muni coaches and leaping out while the bus is rolling, much to the alarm of city officials.

The rash of bus breakouts has infected the 14L-Mission Limited in particular, which saw 10 escapes in the past nine days. Passengers also vaulted from the 38-Geary, 8X and 9-San Bruno lines - and officials have no idea why.

While publicizing the spree Monday, Municipal Transportation Agency officials could only hazard a few guesses as to why riders wouldn't just wait for the next stop.

One theory: Riders who mistakenly get on a bus with limited stops simply grow impatient when they reach their preferred intersection.

"I can't say it is a deep thinker who would have performed that action," said transit agency chief John Haley.

Officials also don't know why the crime, which has happened 15 times since May 30, is so suddenly a problem.

Before the recent surge, the last reported incident of a rider leaping out through forced-open back doors was three years ago and "an isolated incident," said Muni spokesman Paul Rose.

Interviewed Monday, regulars on the 14L weren't puzzling over the bus-jumpers' motivations.

"Sometimes they want to get off and the door never opens or it takes too long, and they want to show off a bit," said Michelle Lopez, 12, who rides the route often.

The girl added, "It's mostly gangsters and sometimes hobos."

The riders may have figured out how to trigger a safety mechanism that makes a bus stop when its back doors are wrangled open while traveling at a low speed, said one 14L operator.

"They just jump off. It's what they do to get off the bus," said the operator, who didn't want to give his name because he wasn't sure he was authorized to talk about the problem.

If a rider manages to get the doors open while the bus is moving at a higher speed, the bus keeps moving and riders can hang out the side of the bus, the operator said.

"I've heard from other drivers that the kids call it 'riding the surf,' " he added.

Rose said Muni buses are programmed to stop when the rear doors are opened and the bus is moving slower than 3 mph. Operators also get a notification on their dashboard when the rear doors open.

Police officials said there was little fun in the bus jumps. The incidents are costly, dangerous and "possibly an act of malicious mischief," Deputy Chief Mike Biel said.

Every time a rider shoulders open the rear doors, the bus has to be pulled out of service immediately and inspected for damage.

Riders are stranded, and the agency has to pay overtime for extra bus inspections. Only one incident caused lasting damage, when a window on the rear doors was shattered, Haley said.

The damage could be worse for the jumpers. Three of the reported incidents happened while the bus was stopped, but most occurred while the coach was in motion. Muni buses can roll up to 25 miles per hour.

"It's only a matter of time before someone gets hurt," Rose said.

Muni and police officials said they could not disclose details about riders who force open doors until after they review surveillance tapes from the 15 incidents to see if a group of repeat offenders might be responsible.

Biel said it's possible the jumpers are trying to escape after doing another crime on the bus. One incident in the transit agency logs says a bus jumper broke out after assaulting a passenger.

Police plan to increase their presence on the bus line using plainclothes officers, Biel said.

"The police department is taking this very seriously," Biel said. "Tampering with a Muni bus while in operation is a criminal offense."