The only time I've seen this happen is if the host has lost connectivity to the device backing the boot filesystem (which does seem to happen randomly in vSphere 5.5). In this case the host doesn't write back that it was put into maintenance mode, so when it reboots it behaves as if there were a power failure and immediately exits maintenance mode once it's back up. This may not be applicable in your case, but I thought I would share.
Thanx for replay....
I was hard rebooting host from iLo. Hence it was behaving as if there were a power failure.