What is the fitna that necessitated the covering of a woman's face, according to the agreement of the imams, when it is feared?

The 'fitna' (trial/temptation) here is that a woman is exceedingly beautiful, causing men to be tempted by her, or that she is feared to be targeted by wicked individuals. Therefore, she must cover her face and hands if she fears 'fitna' because of her or upon her.