Help with sudden spike in read requests

[Subroto Barua]

In our production cluster, we observed sudden spike (over 160 MB/s) in read 
requests on *all* Cassandra nodes for a very short period (less than a min); 
this event happens few times a day.
I am not able to get to the bottom of this issue, nothing interesting in 
system.log or from app level; repair was not running
Does anyone have any thoughts on what could have triggered this event? Under 
what condition C* (if it is tied to c*) will trigger this type of event?
Thanks!
Subroto