Well, partially I do think that NDVO is busy with the student visas. However, my theory is : since max number of ita were issued in December and January, a lot of applicants submitted theirs in Jan and Feb. So, considering the extensive numbers, may be some apps were sent to Ottawa to be processed, for work load distribution. And may be cic just chose a date - say applications submitted mid February onwards would be processed in Ottawa. May be that's why people from February end and March beginning are seeing movement in their apps earlier.
Ofcourse , this is just a thought.