"New Tab" would be further tabs showing webpages trying to open up on your Internet browser. My guess would be that either you are trying to download from dodgy source that wishes to open up loads more ad type windows, or your PC may have an infection of some type. Of course it may be simply corruption or something I suppose.
Does it occur no matter where you download from ?
It may be of benefit to run AV and spy/malware detection apps just to feel ok about your PC.
If that turns up nothing I'd suggest trying a restore from prior to when you first started noticing the problem.