I am not completely sure as of why it happens. I guess, it might be related to whenever there is some update going on the particular token contract address. So, the metamask must be refreshing that token information so as to receive the latest one.
One more thing is, the tokens don't actually go from your ethereum address, just their information disappears from the wallet listing. If the tokens are really gone from your address, you won't be able to recover them this way.
Thanks - I recovered them..... Stupid question.... but - Where are the tokens actually kept? I have a MyEtherWallet but my tokens don't appear in there..... Do you know how to get the tokens into there?