Why, the Trolley Bus Fairy does it of course.

She hovers above, flitting her sparkly, gauzey wings, and gently guides the bus along its merry way, so all the good workers get to their offices on time.

When she's feeling mischievous and wanting to stir up a little trouble, she gently prods the trolleys with her dainty toe, and the damn things fall right off!

I can't answer Mike's question.

But I can contribute another. Is there any substance to the (probably urban legendary) tale that a pedestrian was once decapitated by a wildly swinging trolley-bus pole that came off the wire?

I can't explain the details of the mechanism, but it has to do with the speed at which the trolleybus approaches the switch. That's why depending on your route the driver may or may not slow right down before the switch.

Sorry to be more prosaic, but I have always understood that it is to do with whether power is being taken from the wires as the trolley goes over a sensor before the switch. If power is being taken the point switches, if not it remains in the default position. You see signs saying e.g. "Power right". This explains why the bus will sometimes accelerate as the driver applies power to trigger the switch.

I don't know about decapitation but about 20 years ago someone was killed on Lambton Quay by a swinging pole. I have been on a bus when the pole came crashing in through the back window and someone sitting on the back seat was cut. When the pole comes off and gets caught in the wires it can have a catapult effect which explains why you sometimes see severely bent poles.

For Alan-

The guy killed by the errant pantograph on the trolley bus was walking down the street (Lambton Quay, I think) in front of a mate of mine when it happened.Stone cold.
Oddly enough he was one of two brothers - the other one was a jeweller who was killed in an armed robbery gone wrong.
My folks knew the parents - they only had two boys and both were taken by unusual circumstances. Very sad

This left by 'tonyr' over at MiramarMike:

It depends on whether the driver has his foot on the accelerator or not. If the bus is drawing current, a relay connected to a short segment of trolley bus wire ahead of the junction pulls the trolley wires over to the side, so they line up with the changed direction. If the driver has his foot off the loud pedal, no current is drawn, the relay does not pull the wires over, so the bus can go straight ahead. Of course sometimes the drivers get it wrong and all gets twisted up....

Wikipedia explains it better than I did!

Thank you all ... I now know the answer.

depends on which pedal is stepped on.

break pedal is for bus to stay on main wire(main route)bus is off-power,accelerator pedal is for bus to go through branch route,bus is on-power,the switch which splits the wire is actually magnets,when bus is off power,the poles remain on the wire(no electromagnetic induction).when bus is on-power,the poles are shunted to the branch wires.