It is a reaction bite more than anything. The males are concentrating on spawning, which requires being on the bottom. The females are looking for warmer water before they spawn, then after the spawn, then on goes the feed bag. The water warms first in the upper 10fow typically. So the females suspend during the sunny days in that warmer water. IF conditions are right. The females typically are not all that interested in feeding till after the spawn.
Rivers are both males/females on the bottom to avoid as much current and they haven't put on the feed bag till after the spawn. When the river water warms, is when they start to suspend. IMHO
But I still get ALOT of walleye off the bottom trolling thru out the season after the spawn has happened.