Here's why:
Interesting article, thanks. You learn something new every day!
This was a really helpful article for me, I did some new exercises (with guidance from a fitness teacher) a couple of weeks ago, practiced them a bit too enthusiastically, I think, and 3 days later I could hardly walk downstairs! It was quite alarming but the good old Internet reassured me that it was DOMS and as it followed the predicted recovery course I assume it must have been!