Can't drag and drop video to timeline. Trying to do so creates a new timeline.

So I've been working on a project for a few months now and for the majority of the time things have been going pretty smoothly. However in the past week this bug has cropped up.

I'll grab the video from the source window and try to drag and drop it into my timeline. The cursor does it's typical thing with the grabbing hand and the plus symbol on it. However when I let go to drop it, it creates a new timeline. I then have to copy and paste from there. It's maddening.

Am I donig something wrong, or is this a known issue?