What is the reason for "random" spikes in amplitude of a guitar waveform? In the image below there is the waveform of a clean, strummed guitar note, where the absolute value of the amplitude values has been taken.

At the point marked there is a momentary spike in amplitude before the continuation of the downward trend (there are a few other points like this in the waveform). 

I was wondering if anyone knew how I can describe the reason for this? I assume it's just to do with the random nature of playing an analogue instrument - sometimes the string is going to vibrate in such a way that amplitude isn't going to strictly decrease in amplitude through time.
The strummed string is exchanging energy, not only with the atmosphere and internal heating, but with the guitar's bridge, nut, body and other strings.  Those sources can then resonate and exchange energy back to the strummed string, modulating the vibration's amplitude. This is part of what makes an actual stringed instrument sound more interesting that a simple ADSR waveform synth.  The effect can be even stronger on a 12-string guitar or a grand piano.