Page X has N-tabs, where 1 is the default one.
When moving from Page A to X, all the N-tab-logic (data source MF's) will be executed initially, while you see just tab1 and you don't need the other tab logic executed
This is probably a known "design decision" coupled to a ton of other things, but it becomes a too expensive setup when thinking to usecases with many potential, concurrent users on pages with many tabs (with datasources).
I think this only happens for editable dataviews in a tab, for non-editable dataviews tab logic is only executed when the tab is selected. I've created a ticket for this once, and this was the rerponse from Mendix, which I can agree with:
"This is intended behavior. Tabs are not lazily loaded if they have editable content because this would make it impossible to show validation feedback consistently. ”