Why would adult protective services be called?

Adult protective services were probably notified because the one with the strokes is not able to care for himself. Sometimes, suspected self neglect is the reason why authorities get involved. The hospital had a duty under the mandated reporter law to contact adult protective services. I do not think they would be investigating the brother.

Self Neglect is a reason APS can be contacted.

The hospital called APS

Lor2005,, to whom or what house was your SO released? Into your care or his brother’s? Your profile says you’re disabled and not able to care for him, so where did he go?

Sounds like it's the brother who called APS on behalf of your SO. Therefore, I would think APS is investigating you and your SO for releasing himself early from the hospital with such significant health issues, and against medical advice *AMA*
APS is designed to look into helping people who may present a danger to themselves by being incompetent to make healthy or wise decisions. They'll want to see how he lives, the conditions in the home, etc.

Good luck!