This question needs to be answered very careful.

Better not to criticize your boss or former company because this might leave the impression they have a hostile you. Quite simply, they will sow the conviction that you are a person tongue lashing. You can tell them about your desire to look for the best opportunity, so leave your last job so give the same time to look for a new job without causing suffering former boss during this search. You can also be honest and tell them that he had been terminated your contract because the company re-established or re-construction of the administrative pyramid.