I have a standard white iceberg rose bush that without fail every single year will produce a single flower with a varying degree of red in it. From just one petal, to almost entirely red, to a beautiful half and half red/white. This year for the first time we also had another rose bush of unknown species, colour red, produce for the first time a single split white and red flower. This rose bush has been in the ground more than 5 years if not 10 and has never done this before. Our garden is full of many roses. It is also frequented by native Australian bees.

What could be the cause of this? 

 Location, Orange NSW AUSTRALIA.