The 281e quad function generator is sequencing the beats. Its four function generators are cycling at different rates, sending cv's to affect each other's attack and decay speeds. Each of them is then triggering the percussive sounds. This becomes the rhythm. However, in this case it ended up creating the fills quasi randomly.

The 281e also sends pulses to the 250e which sequences the melodies, starting at 0:55.

A tiny turn of one of the knobs of the 281e, and the percussive pattern would change completely. Maybe I'll keep this patch and just tweak it a bit for another video.