What is the reason for "random" spikes in amplitude of a guitar waveform?
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.