I dunno. It's so strange. Here's a quick tap on the '4' key
1st burst:
2nd burst:
3rd burst:
Another quick tap on '4'
1st burst:
2nd burst:
3rd burst:
Looking at it again there is some consistency. Yay! I see now that over and over again the second and third busts look identical, and not just on the two waveforms I posted. It's not an answer, yet but, I think it is a foothold.
Thanks @jremington
p.s. Even so there sometimes is only one burst and sometimes four bursts. 