Anuj, the main problem I see (and I may be wrong of course

) with your box squat is that you are standing directly over the box, thus forcing you to sit straight down on it and not sit back.It seems to me that your knees break first and they shoot way forward as a result.
I would try stand furthur in front of the box and sit back on it, should be able to get more out of your hammys that way.I bet you could handle EVEN more weight on them cos that looked fairly easy for you.
Anyone feel free to tell me I'm talking out my ass but thats how it looked to me.