This behavior is due to the way content widgets(News, Events, Blogs) filter the displayed content. Currently both event widgets use the url to filter what should be displayed. Please take a look at this forum thread
which explains in detail the functionality implemented. In your case, if you want the Master widget to stop filtering and always display the custom selection of Events you'll need to go to the widget's advanced settings -> ControlDefinition -> Views -> (name of list view) -> AllowUrlQueries
and set this to FALSE. This would stop the widget from filtering and the one that you didn’t click will stay in master mode (list).
the Telerik team
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