The gap between The Walking Dead's ending and The Walking Dead: Dead City left for room for numerous developments to occur offscreen. When last seeing Negan (Jeffrey Dean Morgan) and Maggie (Lauren Cohan) in The Walking Dead, they seem to end on neutral terms. Most notably, Maggie's anger toward Negan is supposedly lessened. Plus, audiences also get a glimpse at Maggie's leadership over Hilltop as it starts being rebuilt. This and Negan's pregnant wife, Annie, suggest some kind of civility between Maggie and Negan will take place.
https://screenrant.com/what-happened-between-walking-dead-ending-maggie-negan-spinoff/