Phantom Gear shifts

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I have noticed on several mountain bikes that the gear cassette will shift on it's own. It happens on mine and several others, with different degrees of shifting. It seems to happen most often climbing or just following a shift. It may just shift 1 cassette, maybe more. Usually the shift is toward a larger cassette. I've checked the troublshooting section in a number of maintenance books and don't see this malady listed.

Any thoughts as to the cause or cure will be appreciated

I have noticed on several mountain bikes that the gear cassette will shift on it's own.

Define several?  Are these bikes all maintained by the same person?  

A perfectly tuned bike won't phantom shift - MTBs however take a lot of abuse.... a bent derailleur hanger will cause the derailleur to not line up properly and you can drop gears.... also there could be too much/not enough cable slack.  If they are new bikes you are talking about, it is probablly just break in cable stretch - and they need to be adjusted.

I'll check the cable adjustment. Thanks