I'm guessing it's closing for auto shutdown protection any CMC message? I remember on the classic it was easy to cause an auto shutdown due to a rapid EGT rise by placing tto much demand on the bleed system all at once, so I guess you need to search for what's causing a potential Auto shutdown, is it a clapped out APU?
The classic reasons:
- Low oil pressure with RPM above 95%
- High oil temperature
- Cooling air valve fails to open
- High EGT
- Overspeed, 110% RPM
- Excessive pressure drop across APU inlet screen
- Duct failure overheat
- Loss of airplane battery power
- Loss of APU sensor signals: EGT, Air inlet temperature, Oil temperature, Oil pressure