Mitchaa,
The only thing I would say about your reasoning is that you are assuming that if the head was installed on 27th March, it hadn't been disturbed since. We don't know the full maintenance history for the aircraft. It surely is possible and plausible that the system had been disturbed between 27th March and the fateful day.
The ASB issued by Airbus covered only the suspension bar attachment and fittings:
Check for correct installation of the suspension bar assembly:
- correct fitting of the lower and upper pins
- presence and correct position and condition of the locking pins
- check the tightening torque of each of the attachment bolts of the MGB bar fittings as per MMA 53-26-00-212 and correct any discrepancy according to the task. Measure the values in accordance with Figure 2.
It was the EASA AD that introduced MGB filter chip and HUMS checks.
Once again, I'm not suggesting that suspension bar failure or error is the cause, but it's possible. Just because it hasn't happened before...........