What could cause Command-Tab task switching to get "stuck"?

When I Command-Tab to task switch (in both Mavericks and Yosemite), occasionally (maybe 60% of the time) the task switcher will behave as if the Tab button was held down. That is, the task-switcher selection will fly over to the right-most icon. If I mouse-over the app icons, the cursor will continue constantly moving to the right. I doubt it's a stuck Tab key, because I use the Tab key all the time when coding, navigating Cocoa controls, etc...never have a problem with Tabs being constantly entered. This only ever occurs on Command-Tab. Any ideas?