Someone is attacking my battlefield and I have the legend Ahri. My Ahri legend ability gives an enemy unit -1. If my opponent uses Ride the Wind to move to another battlefield, does my Ahri ability trigger again?

Yes, your Ahri legend ability will trigger again if that unit attacks a different battlefield you control.

According to the RiftJudge FAQ (ID 4675), Ahri's ability is a Might reduction that triggers when a unit gains the attacker designation at a battlefield you control. If a unit attacks one battlefield you control, receives the -1 Might reduction, and then uses Ride the Wind to attack a second battlefield you also control, it will trigger Ahri's ability a second time, resulting in a cumulative -2 Might reduction.

Key points to remember:

• Per Combat/Per Battlefield: Ahri's ability triggers for each unit the first time it gains the attacker designation at a battlefield you control during a combat. If the unit moves to a new combat (a different battlefield), it is considered a new instance of attacking, allowing the ability to trigger again.
• Not a "Debuff": As noted in the FAQ, this is a Might reduction, not a "debuff," so the effects stack.
• Once per Combat: If the unit were to leave and re-enter the same combat (the same battlefield), it would not trigger a second time (FAQ 8424). Since you specified the unit is moving to a different battlefield, it is a new combat, and the trigger applies again.