Things have changed in the clubroom during the duration of Yuuta and Rikka's school trip, but if this isn't evidence of some kind of seismic shift in the universe itself then perhaps Dekomori's getup and behaviour is.
Following on from that whole Mori Summer web site incident during the aforementioned field trip, it seems that Dekomori has gone further down the path of getting to know this (supposedly "real") Mori Summer, and her garb and behaviour is on account of the fact that they're about to meet. Despite Nibutani's desire to shed this delusional image of herself, she isn't ready to give up that name to an imposter just yet it seems, leading to her following Dekomori to see what this fake is all about.
To give this Mori Summer some credit she certainly looks and sounds the part, and having destroyed all evidence of her past life there's not a lot that Nibutani can do to prove that it's her whose the real deal. However, something is decidedly fishy about this new Mori Summer, and even Dekomori - for all of her loud proclamations - seems to be desperate for Nibutani to do a little more to fight her corner. Eventually, and thanks to some foresight on Kumin's part, Nibutani is persuaded to fight - not so much for her namesake as for the good of Dekomori, as Kumin's suspicions about the true intentions of this Mori Summer prove to be well-founded.
Predictable though it might have been, this was still a reasonably fun episode of Love, Chunibyo & Other Delusions - the relationship between Dekomori and Nibutani was pretty well played throughout, there was a funny line or two to be enjoyed and its ending was oddly sweet in a way. There's still a certain suspicion that the series is continuing to spin its wheels with no real ultimate end goals in sight, but this was at least some enjoyable fare in what has been a rather scatter-shot second season so far overall.