There is 2 Cleanse priority.
High priority cleanse > heal > normal cleanse.
High priority cleanse = Debuff that lose control like sheep, fear, Hammer of Justice... CR always try to cleanse healer first (because they in turn can cleanse the rest)
Normal cleanse = the rest Debuff like root, snare, disarm, dot... CR always try to cleanse the unit have the most Debuff first.
Tip for faster cleanse: Turn off normal cleanse (set to zero) or only start cleanse when unit get very high number of Debuff (9 for example) combat routine will cleanse less often => you have more chance Cleanse not on cooldown => faster cleanse High priority Debuff when needed.
3am here, good night Klickster