Why can thaqulat be feminine with mawāzīn?

Supplementary Practice 26 — Comparative Rhetoric of Sūrat al-Qāriʿah and Sūrat al-Tīn

• Mawāzīn is a feminine singular active participle
• Man is always feminine
• It is caused by an oath wāw before the verb
• Mawāzīn is a non-rational plural that can take a feminine singular verb