Is there any rule explaining why certain second-declension nouns and adjectives with a nominative -er ending drop the e when declined (e.g. ager, liber, pulcher), and why others keep it (e.g. puer, socer, līber)? (A fuller list of these nouns.)
That it has to do with a phonetic pattern seems to be contradicted by phonetically similar words falling in either category, e.g. adulter, adulterī vs. culter, cultrī.