Maybe it's an alcohol related thing, no alcohol..good steady playing, a lot of alcohol..playing speeds up because musicians 'think' they can play the tunes faster, too much alcohol.. the tunes slow down again because the drunken musicians can't relate to their instruments any more!
I don't know if it's more difficult to play slowly, as Smokey suggests, but playing fast can mask a lot of the inability the musician has to play the correct notes.