Quoting from a Scientific American article at
http://sciam.com/print_version.cfm?articleID=63B8AE9D-E7F2-99DF-317460EABD11302F
"This thermosensitive period occurs after the egg has been laid, so sex determination in these reptiles is at the mercy of the ambient conditions affecting egg clutches in nests. For example, in many turtle species, eggs from cooler nests hatch as all males, and eggs from warmer nests hatch as all females. In crocodilian species—the most studied of which is the American alligator—both low and high temperatures result in females and intermediate temperatures select for males."

I seem to recall one theory for the extinction of dinosaurs associated with the collision of a huge meteor and the world, possibly in what would become Mexico's Yucatan peninsula. The debris thrown into the atmosphere may have led to a period of cooler temperatures as the dust in the atmosphere shielded the earth from the sun. That could leave a generation of same sex dinosaurs. Huge volcanic eruptions have done that on a more modest basis. 1816 was "the year without a summer" after Krakatoa's eruption.

http://www.guardian.co.uk/science/2004/apr/29/science.research

The debate about what can alter sex ratios among aquarium fish has long raged on e-mail lists and at aquarium gatherings. Most of the evidence is anecdotal. pH does seem to effect the popular kribensis, a West African cichlid.

Some fishes seem to throw more of one gender when they are young and the other gender when they are old. Temperature is again considered as an issue. Among fishes where females (though it could be males ) are generally a lot smaller than males, haphazard feeding of young adults will lead to cannibalism and too many of one gender. Shelter could be a factor in the same way. Water chemistry, among other things, has also been dragged into the fray. Patents have been issued, by the way, for methods of altering the spermatozoa of mammals in order to control gender.

Hypoxic water (low in oxygen) in polluted areas may also effect gender ratios. Seasonal temperature fluctuations (high and low) have been associated with seasonal gender preponderances among some ocean dwelling fry. Pollution of medicines flushed down American toilets may have a hormonal influence on small mouth black bass males developing ovaries in the Potomac.

http://news.yahoo.com/s/ap/20060907/ap_on_sc/intersex_fish

They have even tried to raise all male Tilapia for purposes of getting more food out of a pond. Efforts to control released triploid (they have three sets of chromosomes, not just two) grass carp by releasing sterile or one gender have been tried - not always successfully in that they have sometimes reproduced..

Now it is possible that your guppies just defied the odds and had all females. If you play lotteries, either don't buy tickets or really do participate. ;)

What age were your guppy parents when the fry were conceived and born? Would the parents have been
under six months of age, 6-12 months, 12-18 months?

What was the usual temperature of your aquarium before the births and for a couple months afterwards?

Was the water particularly soft or low in mineral content? Or was it mineral laden? (So called softened water, run through a basement water softener actually is long on sodium.)

If you (like so many of us from time to time) let water changes slide, would the pH have gone acid?

By the way, might there be some late blooming males among your youngsters? Do all of them have obvious gravid spots? ;)

I don't know that this amateur survey will provide any answers. Your sample is statistically small too. Sometimes we see it situation and "It happens" is the best we can do to describe it. :)