Interesting topic, and frustrating issue in aquaria. I currently have 8 running tanks, and have issues with cyano in only one, despite the substrata, maintenance routines etc. being similar in all tanks.
I did have issues with cyano in the initial period of 3 tanks (first months after setup), but never in the other 5 tanks. After a while, it disappeared (not with much effort), only to remain present although not too much, in a single tank, which has now been setup for 2 years. I have not detected seasonality in my aquaria.
Other notes- A number of cyanobacteria are very beneficial. Spirulina belong in this group, and as we know it is widely used in aquarium as well as in human and animal feeds.
In natural habitats cyanos disperse by various means, most importantly as tiny particles via wind (see StefenF excellent post above), thus colonizing widely separated bodies of water and other wet surfaces. It is unlikely that municipally treated water is a significant agent in transmission. On the other hand, it is commonly observed growing in wet areas on roofs (tiles and shingles) and other surfaces, likely brought by wind. They are also able to survive (in dry form) for veeeery long periods of time, only to 'awaken' when favorable conditions return.