Has anyone figured out what causes us to have sudden random bursts in damage on some bosses?

Example: Last night, I was running Marksman on the Prince Hylas fight. When Duke came out I tossed a Barbed Shot on him, when the tank picked him up my barbed shot dot went off. Instead of seeing 250-300 ticks it was ticking suddenly for 600 a second. Later in that same fight, fired off a Rapid Fire shot that hit for five 1400 hits. I normally hit for about 750. It's all fine when it's working in our favor but that attempt ended when a Soul Fracture hit the raid for 8k.