Probably that happens because MC is building the thumbnail pictures in the background. For getting the images MC accesses the video files through your installed DivX codec. Normally the thumbnail building process starts when you browse the video section inside MC. This behavior should stop when all thumbnails are created.
You can also prebuild the thumbnails manually:
Tools > Options > Tree & View > Thumbnails > Build Missing Thumbnails...
However, this thumbnail building process should not steal focus (at least not from MC itself). It should only slower your browsing experience slightly. I would recommended uninstalling DivX and using the
CCCP codec & filter pack instead. In my experince the Direct Show filters included in the CCCP pack are much better.
Check my answer in your other thread for more info.