I've seen this too... one day for no good reason, my heart rate goes sky high even though I'm not doing anything different.

I've worked through it and it no longer seems to happen so my best guess is that it's something we experience in the early stages but it goes away as we become more conditioned.

I've spoken to some friends that are really in shape and they say that this never happens to them. It takes pretty much a consisten effort to get them to a high heart rate. They never seem to see abnormal spikes.