If it now only ever happens when the microwave is in use, that obviously goes a long way to pointing a finger at the microwave. However, any fault which may exist in it must be a very marginal one, give how infrequent are the trips. Furthermore, many/most appliances (probably including microwave ovens, but probably not a conventional oven or conventional hob) which contain power supplies or electronics (and some others) result in a small degree of live-earth leak (not enough, in itself, to trip an RCD) which raises two possibilities (in addition to the possibility of the microwave actually being faulty) ...
.... Firstly, if, by chance, several such appliances are being used simultaneously, each producing a small L-E leak, the combination of all those little leaks may become enough to trip an RCD. That's why it is important that you try to ascertain and record all of the appliances/loads in the house (well, on the same RCD) which are 'on' if/when the next trip occurs.
.... Secondly, it's possible that you have an RCD which has become somewhat 'trigger happy' and which therefore just occasionally trips in response to just the small ('normal') leak produced by the microwave. It might therefore be worthwhile to get an electrician to check the 'trip threshold' of the RCD.
Kind Regards, John
