Why does the tanwīn of يَوْمَئِذٍ take different rules in verses 4 and 6?

Al-Qāriʿah and Az-Zalzalah — Comparative Tajwīd Practice

• The tanwīn moved position
• The ending vowel changed
• The verses have different revelation locales
• The next letter is tāʾ or yāʾ