Take for example Google Material design website, that has big sub-menus in the sidebar's menu.
The sub menus are closed on page load.
Material design Motion Style Layout ...
Clicking on a menu title expands its sub-menu
Material design Introduction Environment Material properties Elevation and shadows What's new Motion Style Layout ...
When clicking on a different menu title, should the opened sub-menu collapse or not?
Material design Motion Style Color Icons Imagery Typography Writing Layout ...
Material design Introduction Environment Material properties Elevation and shadows What's new Motion Style Color Icons Imagery Typography Writing Layout ...
In Option A (which might animate) the user's cursor would be over some different element after the sub-menu collapses (not good). Displays the menu in a cleaner way, without the need to scroll.
Option B does not trigger undesired actions (Option A somehow triggers an action). The user might have to scroll to see the just-opened sub-menu. The user might have to close the opened sub-menu to see clearly what he just opened.
What is the user's expectation, which option facilitates user's task?
On a side note I just read this on Navigation patterns - expanding navigation drawer:
Selecting a collapsed section expands that level in-line, hiding all levels outside of it.
This looks like Option A. However this is not applied in the same website, that applies Option B.