MAPPA stayed as the studio, so there wasn't a switch or anything. What really happened was more of a scheduling and production issue, because the second season came out way faster than the first one, and that really affected the quality of individual frames. The first season had more pre-production time and the directors were able to detail the fights way more, whereas in the second one the studio got pressured to deliver quick and ended up prioritizing keeping the project on schedule instead of polishing each scene.
Plus, it changes a lot who's directing the episodes and who's doing the storyboard. The first season had really well-known names in the animation industry working on specific scenes, and that makes all the difference. In the second one, there's more variation in quality because there are more people involved and not everyone's at the same level. It's not exactly a budget cut in the sense of "there wasn't money," but more like not having enough time to redo scenes, fix details, and keep everything as clean as before.
This is pretty common with popular anime, you know. The pressure to release early kind of kills the perfection you see in the first season, especially when the anime gets that unexpected audience boom.