Ooi should take Kashima to make Kitakami jealous and make her realize she's been taking Ooichi for granted

OK, now it's my turn to ask. What is the relation(ship) between Ooi and Kashima that various artists have been pairing them recently?

AFAIK, it's a senpai-kouhai one. Both were training ships before Ooi was converted into a torpedo cruiser.

But why recently? The Mitsukoshi collaboration might has something to do with this.