Are you all playing on computers with Windows 10? I had this happen to me frequently last week, apparently there was a glitch that got implemented with the major update for Windows 10 they released about 2 weeks or so ago, which all computers running Windows 10 forcibly applied. And this bug would happen to more than just League, it'd sometimes have something weird happen in my YouTube videos whenever I went full screen where the taskbar on the bottom of the screen would not fade into the background like it should've. Anyways, they've now since fixed that bug with a newer update...that for some reason Microsoft decided didn't need to be implemented right away, because reasons?
So if you are running Windows 10, I'd try going to the Microsoft website and looking for the link to download the latest version of Windows 10. Check to see if it matches your version of Windows 10 or not...ugh I forget the exact path for that atm. Hold on...there it is, go to the Windows Menu and hit System Information. It should be the 3rd item down on the first screen. If they aren't the same, download and run the new version. It'll have to restart your computer and stuff while it's going. And personally, I had to restart again for it to work properly after it got done.
Hope that helps.