From the combination all the plot lines.

It seems to me that Kagha wants to close off the grove so that the shadow druids can have an easier time taking over the grove AND keep safe from goblins. They don't want the tieflings maybe partly due to what happened before with the town descending into hell (these are the tieflings that were part of that.) The druids were conflicted in keeping the tie flings because they just became leaderless and Kagha was a strong figure to band around to (This type of stuff happens) They are leaderless because the goblins captured Halsin. Halsin said that the goblins were causing an imbalance. So as a druid, killing/stopping the goblins is actually GOOD.

The tieflings were bringing in goblins attacks. Thus the tieflings were also causing an imbalance. So Kagha, a member of the shadow druids, decides to close off the grove. If she really was 100% committed to shadow druid ideology, she would've just killed the tieflings herself, but she was trying to radicalize the grove into shadow druids.

Simply put

Refugees from Hell have come to reside in our grove after a pretty large incident concerning Hell.
Leader taken by Goblins
Goblins are attacking trying to kill refugees.
Shadow Druids try to fill vaccum- Blame refugees for goblin attack
PC can either help tieflings or let them die.
Kagha don't care as long as they are gone before they close off the grove so she can take control

Shadow Druids are pretty harsh when it comes to anyone messing with nature. Now imagine a Hellspawn (Which already get bad rep) taking one of your relics. They gonna be pissed.

I don't really understand the disconnect here.

If a certain race of people with a bad rep come to a grove that is being taken over by extremists that believe anything that upsets with nature and balance should be destroyed or leave then I don't find this arc to be too unbelievable personally.