The placement stats are calculated as the ratio of number of students placed to the number of students who sat for placements.
A lot of students have gone for higher studies also, and many are preparing for competitive exams like UPSC/CAT/GATE.
But these factors are no direct reason to justify the fall in placements for sure.
This year due to economic slow down, the IT companies are slashing their employee base. Infosys, Wipro and TCS have already slashed thousands of their employees and startups have slowed down their recruitment drives.
You must’ve heart about the famous debacle of Flipkart at IIM-A for delaying the joining of their recruits which resulted in its blacklisting from various IITs and IIMs.
The decline in Placement stats is also a result of the fall of the IT boom which India was enjoying currently.