The Imams عليه السلام know when they will die and they die voluntarily

8. A number of our people has narrated from Ahmad ibn Muhammad from Ali ibn al-Hakam from Sayf ibn ‘Umayra from ‘Abd al-Malik ibn A‘yan from abu Ja‘far عليه السلام who has said the following. “Allah, the Most Holy, the Most High, sent support for Imam al-Husayn عليه السلام up to the fill between the heavens and earth. Then he was let to choose either victory or meeting Allah. He, however, chose the meeting of Allah the Most High.”