the how of getting these is mostly unknown and changes... it would seem that certain geography is more prone to them. The general explanation is that marketing targets those that would most likely take advantage and the offers are targeted to get heads on pillows at certain times of year.
It is also hypothesized that the offers are timed and sent to folks in areas where WDW saturation is low to drum up buzz... if you neighbor starts going on and on about how cheap they got to goto WDW maybe a keeping up with the Jones affect will occur.
A lot of thought has gone into figuring it out on other boards by a lot of smart people... it is kinda random.