Same for 風向き (かざむき)
Also in the special kanji derived from other word topic there is plenty of example of 目 pronounced as ま (窓=目戸=まど, 瞬く=目叩く=またたく,瞼=目蓋=まぶた), so I wonder if there was some kind of sound change from あ to え long ago, and the あ form only survived in a few expressions.