They didn't complete the encounter, not until 3 DPS were kicked from the group. Which 'supposedly' was a rules violation according to SE. That's why its weird. Even though booting the DPS and getting better ones could potentially make the run a success.
If a tank dies to a tank buster because they didn't use a CD, they can be booted for it. No violation of any rules. If a DPS does too little damage and you use a parser to determine that (since its the only way to do so), and boot them for doing too little. Despite that being just as important and vital as a tank using a CD, its considered 'harassment'.
Of course I use quotes because its still hearsay to me, I've never seen an instance of someone being actioned for this personally.