The CR for the hail of arrows trap is off.
Hail of Arrows Trap CR 9
Type mechanical; Perception DC 25; Disable Device DC 25
Effects
Trigger visual (arcane eye); Reset repair
Effect Atk +20 ranged (6d6); multiple targets (all targets in a 20-ft. line)
The CR is actually 8:
Perception DC 25: a +1 CR modifier.
Disable Device DC 25 : a +1 CR modifier.
Atk +20: a +2 CR modifier
6d6 damage : average damage is 21, this needs to be multiplied by 2 due to the multiple targets, 42/10=4.2, a +4 CR modifier.
1+1+2+4=8.
I think the error lies with the arcane eye, that probably errorounsly triggered a magical trap +1 CR modifier.