The heder-hadar alternation is limited to nouns with final resonant, because resonants are particularly prone to acquisition of syllabicity, which, in turn, often leads to epenthesis. The construct forms of such nouns underwent epenthesis earlier than their absolute co-allomorphs -- early enough, in fact, to be affected by the well-known Hebrew stress-shift -- because they lost their case-endings earlier. The original epenthetic vowel was lowered to a by Philippi's law. The retention of e in havel is due to overlapping of Philippi's law and stress-shift.
"This paper is part of the Proceedings of the Second North-American Conference on Semitic Linguistics. Santa Barbara, California March 25-26, 1974