Can a person die without a cause?

Yes, a person's appointed time of death (ajal) is fixed; it neither advances nor delays. No one can die before their appointed time, as confirmed by Allah's words: ﴿So when their appointed time comes, they cannot remain behind an hour, nor can they precede [it].﴾ and His saying: ﴿And never will Allah delay a soul when its appointed time has come.﴾

So, when a servant's death is due, Allah causes them to die according to what He has decreed for them. They might die without an illness, or in a car accident, or by murder, or illness, or something else. Sudden death is when death occurs without a cause like illness or otherwise.