These nouns are a subgroup of the larger group of Segolate words, which, as noted in the linked Wikipedia article, are conjectured to have descended from older semitic words, where, often, the penultimate syllable would not have taken a segol but a patach. For example, yeled (child) is conjectured to have originaly been *yald.
Indeed, we see this pronunciation reappear when a possesive suffix is added to the word: "my child" is yaldi, with a patach under the yod.
Therefore (and this is not mentioned in the Wikipedia article) we should not be so surprised to see the original form re-assert itself in the pausal form, with the archaic patach lengthening to a kamatz, thus yaled.