I like that theory!
It does seem that many sources describe dragons as being extremely long-lived, so in addition to spending a lot of that time brumating they probably also have a very slow reproductive rate, which would be necessary in order to keep the population density low enough to not exhaust the food supply. Maybe they are like 17-year cicadas, and every 113 years (113 because it is a prime number, it is over a century, and it ends in 13) they all come out at once to breed.
And then every 113 years they go on rampages, which is just often enough to keep the knights interested in making periodic attempts, thus providing the occasional interim mineral-rich snack. Otherwise they might have to gut-load the cattle with iron filings.