The new behavior you are seeing was added in version 2.5 due to performance issues with the full-width treeview when there is a large amount of immediate child records below the top-level parent. Bascially the tree tries to load everything into memory, which can exhaust the available resources in the browser before all records are loaded, or cause the tree to simply never load. This new functionality was a way to work around this issue.
It should only kick in when there are more than 50 immediate children beneath the parent record - for example, a collection full of items, with no intermediary levels. If your content is organized into series, for example, this won't generally happen, because the tree can load the lower levels progressively (as the series will be collapsed when first loaded).
At this time, the behavior is not configurable. If you are a developer (or have access to one), you can see the pull request that added this ability to AtoM here, which will show you which files were altered and how: