2 dimmer modules behaving strangely at the same time

I have a system in my home that has been running with no problems for about 5 years.

The output units are:
4 x 12 Channel relay
4 x 8 Channel Dimmer
1 x 4 Channel Dimmer
11 x Sweep Fan Controller
1 x ELV relay module

I recently had an issue where the loads on 2 of the 8 channel dimmer modules turned off. Checking the switchboard I found that these units had no indication on at all (ie Unit Indicator, CBUS Indicator and all channel indicators were all off). There was power to both units.

The rest of the network was operating fine - loads could be turned on and off, DLTs all behaving nicely etc

I cycled power (opened and closed the breaker) with no apparent effect.

When I pressed the channel 1 local override button on the first faulty unit it seemed to reset the unit - the Unit Indicator and CBUS indicator plus all of the loads that were on when the fault occurred sprang to life.

Pressing the channel 1 local override button on the second faulty unit had the same effect.

A couple of times, later the same evening, all of the loads on the two dimmer units dropped out for a few seconds and then came back on their own.

With two units going down at the same time, it seems that the root cause must be external.

Maybe I was seeing the end result of a bit of dodgy tap changing on the supply side. Is there some sort of over voltage protection built into these units?