In this case is it always the seasoning? As in there is another word for an actual pepper?
This happens a lot with vowels followed by an r or similar consonant where the r is assimilated by the vowel. The vowel structure of the noun also lends itself to the dropping of the "e" in the direct article so over time the pronunciation of the r became less pronounced. It makes sense when you think about how many language favor two or three syllables and the first thing to go in accommodating the simpler syllable structure are the vowels, especially vowels like "a" and "e". Most irregularities of this nature are generally due to the tendency to simplify speech.