Why do the feet have to be tied after one's death?
بِسْمِ اللهِ الرَّحْمنِ الرَّحِيْم
In the name of Allah, the Most Gracious, the Most Merciful
When a person passes away, their eyes and mouth are closed in order to make the deceased presentable and to prevent their appearance from becoming fearsome or disliked. The jaws are tied together to prevent anything from entering the mouth. The hands and feet are straightened as this is difficult to do later once the body is cold and stiff.
Tying the feet is not a must but it is recommended in order to prevent the legs from moving about while the body is being transported for washing and shrouding.
Only Allah knows best
Written by Maulana Umer Khan
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 Raddu al-Muḥtar, 2/193