Here's the thing. They are, of course, your tunes, so when you're running, your body is operating faster, so to speak. Heartrate is up, Alpha and/or Beta waves in the brain are in motion, and because they are your tunes, you expect them to be as fast as your body feels in motion. In your head, the song will play back 'faster' because your body is well, operating faster. So when the song plays and it's not up to the subconscious expectation, it seems slower, and the 'slowdown' will be more pronounced the more intense the exercise is. I know what you'e talking about because this happens to me from time to time, albeit very rarely.
Adrenaline does have a part in this, yes. But it's mostly a mental thing.
Now, as Rozo is hinting to, running with a metronome? I believe the metronome will match your song tempo either way, although I've never really tried it and I'm not sure what the result would be.
And Modus, you're not magical. One of the effects of adrenaline on the body is that time seems to go a little slower. This is usually a part of the fear auto-mechanism ( for example with a poisonous snake: snake hisses and rattles tail, fear rises, adrenaline begins to flow, heart rate speeds up, body's reaction time speeds up, perception of time slows down slightly). Those last two are closely connected, and is affecting the 'tempo' of your own work. The reason only your music is being affected is because, well, it's yours. You "know" how it's supposed to sound in your head (because it's your own creation- you made it from start to finish), and when the adrenaline starts pumping, your internal metronome speeds up but the song doesn't, so it plays back 'slower'.
I think I just reiterated myself somewhere.
tl;dr it's all in your head. your body's chemicals are screwing with you