Tempo issues are a symptom and not a cause, so here's a look at some of the causes.
- Excitement: When we play intense pieces with dramatic dynamics, we can lose ourselves in the music as an audience member would, and let the emotions of the piece carry us away.
When that happens, we go from being in control of the music to having the music be in control of us, and that's why tempos rush or slow down.
It's the same situation a comedian face when they can't stop laughing at a joke they have to perform. They have to get it out of their system in order to perform it, and so do we for an emotional passage.
- Technical difficulty: When we're facing a difficult technical spot, our body goes into panic mode and takes over our mind.
The key here is to use our practice sessions as a practice for our mind to remain in control, by always playing at tempos that allow us to stay in charge.
- Nerves: When our nerves take over during a performance, we stop thinking of the music to think of a million other things instead, such as what the audience is thinking or if we picked the right outfit.
Our fingers are left playing on their own, which impacts the tempo in a big way. The solution is to make sure we keep a cool mind at all times.
Picture from http://www.patentspostgrant.com/lang/en/2010/08/uspto-pilot-program-aimed-at-reducing-ex-parte-reexamination-pendency