While I wouldn't say that it happens every year it is pretty common for this type of flooding to happen somewhere in Athens around this time of year. The rain seems to come in heavy storms which often deposit excessively high volumes of rainfall in relatively small areas in a very short time. When it happens in a hilly area with high urban density the water gets further concentrated rushing through the streets. Having said that, this event does seem particularly extreme. Fortunately we seem to be avoiding the worst weather so far.