Why were there two factions in the town?

Honest question here. I watched this show multiple times, but I can't find the reason why there are 2 factions in the town. What was happening between Boyd and Donna because in the first episode, when Boyd wanted to tell Ellis about Megan's death, Donna told him "we have an agreement sheriff, you don't come here."

It is odd because everything else showed us that they were good friends, so I am just wondering if someone has more insights about this?