Perhaps they thought you were going a bit slow or perhaps they were too impatient. I encounter both kinds of dps and tanks in my groups: tanks that like taking their time marking everything and it feels like a snoozefest and dps who have had way too much coffee and just chainpull everything and not letting a tank mark anything at all. Both are pretty annoying there needs to be a balance of tank being quick to mark and dps having enough self control not to pull everything.