Leg tightness during taper

During my last few taper weeks in preps for big races, I have noticed that there will be a few days in the middle of my taper where I am completely helpless. I will start a workout, get about 15-20 minutes in (it doesn’t matter if it is a ride or a run), and then my legs will tighten up to the point where I can’t continue. No amount of stretching or lowering the intensity/pace will allow me to continue. After about ten minutes, the pain will be gone, but will come back if I try to continue the workout. After a few days, I will be back to normal. Any insight as to why this is happening? Or how to avoid it?

what does your taper look like, in terms of workouts, laid out day by day?