I had to look this one up. The French word trapeze is from late Latin trapezium. Presumably the hanging bar is connected by two ropes to the ceiling, but they are wider at the ceiling than they are on the bar, so it does make the shape of a trapezoid. However, my American Heritage dictionary says that the ropes are parallel. So perhaps it was an optical illusion, namely that because of the distance the higher connections looked closer together than connections at the bar, even though ropes were actually parallel.
Admittedly this sentence is unlikely to be uttered in very informal contexts, but still to me the pronunciation used on the audio "utrapez" sounds a little odd. I would expect to hear "vetrapez" in normal speech. The "u-" pronunciation is no doubt technically "correct", but it seems highly formal.