Hi stm, welcome to the cardio forum. I am sorry to hear about
your grandfather. I cant really speak for what could be causing this to happen as there are many things that could make this occur. One could be medications, or if he has any kind of heart arrhythmias or irregular beats. I would suggest that you speak to someone in your family who would have reliable information on what is happening with him.
Please do keep us updated on how he is doing and know our prayers are with you both. Take care