People who watched up to 14 elsewhere and did a search for 15, finding that one and watching it.
Same is probably true for episode 11, (over 9000 views), though it is also possible some people were just rewatching the Gamilas battle with the Comet Empire as that was rather cool and unexpected. There are also a lot of fans that liked the second series more and enjoyed that battle.
Before ep 11 was up it was already skewed with over (if I remember), with over 3,000 views. I do not doubt that individuals re-watched that particular ep for the battles but also for Melda too, since she was scantily clad and the bath scene to with Yuki and Yammamoto.
No doubt that a lot of fans liked the Comet Empire arc a lot more with all the battles where the Iscandar arc had few battles.