Or is there something which sends MIDI clock or MMC commands (your master keyboard probably)? Then try disabling the MIDI clock slave device in the MIDI preferences.
Does this only happen in the Pattern Editor, or also in other Renoise GUI parts?
There are some people on OSX which have problems with stuck arrow keys, but there the problem is that the arrow keys are not recognized all the time: https://forum.renoise.com/t/osx-cursor-keys-work-intermittently-in-pattern-editor/26485
How are your keys setup for OSX spaces?
See http://www.renoise.com/board/index.php?showtopic=22555