They are called "articulated" busses.

They were also called that about 10 messages ago when I said that

As to why, actually, well, the web could have answered that, if you'd looked in the right places.

One of the problems with collecting power from overhead wires is ice buildup as well as things which cause the wheel or shoe to bounce slightly away from the wire can cause arcing. Sometimes it's more serious than others, and sometimes it just goes on in one place frequently, and the wire burns through and comes down.

A pole can also come off the wire, spring upward and tear down the support wires, bringing the whole thing down. There are other failures.

The wire can stay live after it comes down, depending where the substation is and if it was grounded and tripped the breaker in the substation.

Interestingly a car with heavy current demand can also trip the breaker if you try to accelerate too fast. Our local trolley museum has a smallish sub that can occasionally be tripped by some of the balkier cars when a motorman with a heavy hand or foot is at the controls.

And there are also on-vehicle breakers. At least once as a random rider on one of the older Pittsburgh PCC streetcars which are now retired, the motorman went to pull away and tripped the breaker. He got out, reset it, and tried again. The car moved, and the breaker tripped again.

We were a few hundred yards from the maintenance facility, so they sent down another old car with a towbar to tow that car back to the "barn", and then a few minutes later a car went the other way to a loop up the line a bit, turned, came back, and picked me up for the remainder of the relatively short ride I was taking as a joyride.