For some reason; for an unclear or unknown reason (used to indicate something happens without a known cause).
なぜだか今日は眠くてしかたがない。
For some reason I'm extremely sleepy today.
なぜだか彼女は私の電話に出なかった。
For some reason she didn't answer my call.
なぜだかいつもより道が混んでいた。
For some reason the roads were more crowded than usual.