Possibly a late frost killed the flower buds? Did the ones that bloomed last year bear a lot of fruit? (which can reduce following year's bloom) What kind of pears? (Maybe fruit set wasn't noticed if it's an ornamental pear.) I probably wouldn't say it is common (another question: are they established, or just a few years old?), but I think a late frost or freeze could be a common cause of this in some areas.
