Because she was mistakenly inserting the earphones instead of the smartphone charger.
Because she was mistakenly pointing at the earphones instead of the smartphone charger.
さす (入れる)= to insert (into)
彼女はイヤホンをさしていた = she was inserting the earphones
間違えて = mistakenly
スマホの充電器 = smartphone charger
なぜなら、彼女は間違えてスマホの充電器と間違えてイヤホンをさしていたから
Because, she was mistakenly inserting the earphones instead of the smartphone charger.
指す (指で) = to point at
なぜなら、彼女は間違えてスマホの充電器と間違えてイヤホンを指していたから
Because she was mistakenly pointing at the earphones instead of the smartphone charger.