Part of the rationale has to do with all the injuries, which really affected training camp. We saw Kevin Huerter start right at the beginning of the season even though he had not played in a single game. Trey Lyles has only played in 18 games. Malik Monk missed 7, as well. DeMar DeRozan missed 5. Jae Crowder started mere hours after signing a contract due to an injury to DeRozan.
Another part has to do with a largely inexperienced bench. Like many NBA coaches, Mike Brown does not trust rookies or even second-year players. He has fidgeted around, trying to find consistency, when more patience with the young guys might be advisable. Frankly, he has a much poorer bench compared to his first two seasons.
Meanwhile, the coach has tried to find a "formula" for rotations that works, even up to now, a third of the way through the season. This happened last year, as well. Instead, we get good performances rewarded with DPCDs, bad performances rewarded with starting positions, too many minutes for all the starters, and so on.