Environmental factors such as late spring freezes can kill the buds. A fungus called azalea petal blight can also kill the buds.

We had some dry periods in PA around the time the buds were swelling. Did you get to water yours during that time? The flower abort could have been due to drought conditions as the plant will often let the flowers go in favor of the foliage before the foliage droops.

If you had a period of warmish weather followed by the possible late freeze as mentioned above, your buds probably froze. Flower buds are approx 10F less hardy than foliage buds.

The azalea planted last year is behaving normally. They will often not bloom their first year after planting, channeling energy into root formation and establishment in place of setting flower buds.