I noted one thing out of curiosity that in recent educational round at 90% system testing problem E was solved by arounf 500 users but at 100% it wnet upto 2000 , how is it possible that during last 12-17 minutes so many people got idea how to solve that question link of question
It is because of the order of questions. A lot of participants worked "long" on D before reading E. Then, because E is much simpler, they quickly solve it.
so,you want to say that thinking over D for so long make contestants brain sharper for that time that they solved E just after seeing it.
I find this pattern in pretty much every contest, which is because in the last 10-12 mins all participants submit their solutions regardless of its correctness, which is the reason why there is spike in the number of submissions at the end. Due to so many submissions taking place, some them are bound to be correct. Although in this particular contest E was much easier than D, and people might have realized this later as spookywooky suggested, which might have been a contributing factor.