You may give any of the following reasons::

(1) The projects are not impressive (Learnability is less)
(2) Timings are almost 12 hours daily. Not adjustable with 
those timings daily
(3) CTC is not matching with the work load may be

This Question has various perception. Although there might 
be n number of reasons for a person to leave the company , 
i feel that he/she should not give resasons like work 
pressure,low salary or anything negative about the previous 
company...cos your next employer will not promise u a 
pressure less job or a great hike in the salary and they 
would perceive you doing the same thing to them down the 
line. In my opinion the best way to tackle this question is 
1. Give a good opinion about your previous company
2. This is my best shot...Praise the company your are 
attending the interview for with statistical details and 
comparisons to other companies (be specific and give 
genuine details, better do good homework)and show them why 
you picked them.3. Be frank and open that you are looking 
for a better career prospect in terms of 
knowledge,rank,pay,onsite,exposure (whichever applicable). 
Every body understands that all have some aspirations and 
over a period of time and if it is not met,they look for a 
change.
4. Every employer understands that change is necessary and 
no one would expect anybody to stay for 5 or 10 years bcos 
they too would have seen people leave their company for 
same reasons.    
 This is just my personal opinion..views may differ from 
person to person.Hope this helps...thanks