。
Leaves blew away in the wind.
Literal
Tree-[possessive-の]-leaves [subject-が] wind [by-で] flew.
Here で is clearly causal — 'by means of / because of the wind.' 飛ぶ ('to fly') is used for any airborne motion, not just bird-style flight; leaves blowing around are 飛ぶ, as are paper, dust, and rumours. The contrast with locative-dative に (as in 風にそよぐ) is one of the recurring particle decisions for weather-driven motion.