My permanent crown (molar) keeps coming off-twice in the last 6-7 months. What are some of the cause for the crown to fall off?

There are multiple cause why a crown would fall off. One thing to understand about retention and longevity of dental crowns is that it is not only a use of a right cement, but also, and mainly, a proper design of the preparation of the tooth to receive a crown. 
Yes, It could be a cement failure, meaning cement was not mixed properly or not enough of it. It could be that the preparation of the tooth was not ideal, i.e. not enough mechanical retention placed. Also crown could be placed incorrectly, which is rear. It mostly likely also be a combination of above factors.

Sounds like you're asking for a mini-dentistry lecture. Okay, here it comes.There are basically two issues involved here. One is retention and the other is fit.Retention is related to how much tooth is left under the crown. A larger (in diameter) or longer tooth will have more retention than a shorter (in diameter) or smaller tooth. Retention is also a function of how the tooth is shaped. Ideal is a 6 degree taper, where the four sides of the crown of the tooth are relatively parallel. If the tooth is shaped like a tepee, with much angled sides, then retention will be considerably less. There are other factors here, but those were the most important with regards to retention.Fit is the other major reason why crowns come off. Basically fit is how accurate the crown fits the prepared tooth. Does it rock on the tooth, or is it stable? If it fits completely, covering the entire tooth down to the prepared margin, and there is NO gap between the prepared tooth and crown, and a good cement is used, then it will be much less likely to fall off. Thus, good fit. Dentists have a term often used when crowns don't fit well and there is a large gap between the tooth and the crown that can allow wash out of the cement. "It fits like socks on a rooster" is how they are described, meaning poor fit and large gap.If there is a gap or a poor marginal fit, many times this will also show up on an x-ray. If the dentist can stick his dental "explorer" into the gap, then that is another indication of poor fit. If the tooth is too short, it can be made longer by one of two procedures (or combination of both), a procedure called "crown lengthening" or "root canal, post and core build up". Just go back to your dentist, ask for an explanation as to why he thinks this is happening and what (if anything) he can do so that it doesn't continually happen to you. If you don't get an answer that satisfies you, then seek the opinion from another dentist. If the crown can't be made to stay cemented on your tooth, then the alternative may be extraction and either implant or bridge. Hope your situation can be solved without doing either of those.Good luck!

Crowns can come off but it should be rarely rather than every 3 months. Sounds like you have an ill fitting crown or a preparation that has inadequate retention to hold the restoration (if it is a cemented restoration). If it is a bonded restoration the bonding technique is not correct or again there is inadequate tooth structure to bond to. Your dentist should redo a no cost or find another office which can do a proper restoration for you.