Two possibilities:
1. Someone at the Pentagon wanted to exploit a disconnect between SecDef and POTUS during a timed crisis...
-or-
2. Someone in the SecDef/Pentagon security apparatus was concerned that someone in the White House staff would exploit SecDef being down.
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Or the man went in for a simple outpatient procedure and things went south for no reason. It happens and people don't worry about calling work from intensive care.
Austin is well known for being super private. I doubt he'd announce an elective surgery, he'd just take a couple of sick days and let Riker have the comm.
Just sayin it's also an option.