There is a logo for Wolfram Workbench in the window. When I click it
(is this an IQ test, Wolfram?) the usual security dialogue stating
that Wolfram Workbench.app was downloaded etc appears. When I click
Open Wolfram Workbench opens asking for my selection of a workspace
and the loads.
Do not click it. Instead, drag the Wolfram Workbench logo to your Applications folder.