Normally, during the split phase of Siax when the four Echoes are channeling Caustic Explosion, the detonations occurs simultaneously with Siax's own channel.
With the Sugar Rush instability, this changes - but only on the 33% phase. The 66% phase operates normally, but on the 33% phase, Sugar Rush causes the four Echoes to detonate 35% earlier than Siax's actual timer.
Is this intended? It sort of makes sense as it's a channeled attack and they are veterans, thus qualifying Caustic Explosion for Sugar Rush's speed, but it (a) directly affects a specific mechanic of this specific boss, turning Sugar Rush into a CM for the CM, and (b) does not occur on the 66% phase despite the Echoes being the same in both. Something just isn't right with this instability on this encounter.
Edit: Further VOD review has me even more confused. We spent two hours trying 99CM with Sugar Rush tonight and cleared the 66% phase easily yet the 33% phase wiped us due to faster detonation - but one wipe at 66% also featured an earlier detonation. I don't see a consistent application of this.