Actually Canon recurrence are very few for Flandre, as she is only seen in 2 games for a single fight and there is only one other. The perils of being a EX boss that never is seen again in canon.

For this it would fall into the unknown/headcanoned history behind the Scarlet sisters. When being rescued from the hunters that had captured/tortured her, Remilia got staked right in front of Flandre, sending her into a rage. As Flandre has a tendency for not understanding what she is and what weaknesses or knowing what would really kill a vampire, she decided to use this ability to take down at least a few of the hunters before Remilia regenerated enough to continue to fight and calm Flandre down before anything worse could occur.