Hard to really tell, but since you've tried to identify possible reasons and can't think of anything, I'll throw out what I would look at.
- Ambient temperature or humidity might be making it more difficult on some days
- Air pollution could vary from day to day
- You might not be eating the same amount of food, or the same foods every day, resulting in different energy levels
- You may have had poor sleep the night before, resulting in less energy levels
- You may not be giving your body a chance to properly recover, so after multiple days of the same workout, your body is just saying 'enough'
- You might be preoccupied with something, which may be adding additional stress that your body isn't adapted to handle
It might not be just any one cause, but on some days multiple things lining up to cause trouble. That could also be why you're not able to notice any trends. If any one of those things isn't really enough make it harder on you, it might look like one day you had a miserable night and had problems with your routine, and the next time you had problems you slept very well.