Tero what Pete and Les were on about is called differential shrinkage, it is where one material shrinks at a different rate to another, in your case two different timbers each having a different rate of shrinkage. This works the other way as well, as timber sucks up the moisture from its surrounds different timbers will suck up differing amounts, this expansion and contraction can cause a glue joint to fail or the wood on either side to fail. But you may be lucky.
I would say that although I quite like your platter and looks to be well made considering the lenght of time you have been turning it appears that one of your glue joints is not so good as the others. Apologies if that is not the case.