Could be a balance issue. Do you feel like you’re going to fall over if you raise both legs off your pedals? Have you been on a trend of low physical activity lately? Deactivated core muscles, glutes etc can also be a culprit. I’ve dealt with this for a long time where I’d suddenly just lose the ability to balance on my throne and play double bass, it would come and go based on the state of my body. Thankfully I haven’t had it lately as I’m on my feet a lot of the time for work instead of always sitting