You mean other than Jill serving to cock block the mc one more time?!
Like
sixart said, it's jealousy.
She first baulked a bit when she and Riona caught him with Sage in the garden.
It's not about sex though, it's about his strong connection with other girls that Quinn coverts.
She's beginning to show a side of herself to the mc that she has hidden for so long; if she feels she's no more important to the mc than his other friends, she goes back to self defence mode and severs the link.
It's classic "Bad girl wall building 101".
That, or DPC's logic is:
MC + Jill != Quinn
I haven't encountered that in the game, but seriously having to explain himself other than, "Just stirring up Tommy" is bullshit. If he hands his balls over to the girls now, imagine how tight the grip will get later...
EDIT: Zoey has really done a number on you hasn't she. Lucky it's not a 6 month wait for the Interlude. DPC has assured me it's only gonna be 5 and a half month...