I'm guessing if there was an overpressure event, that one would normally think that it was due to unanticipated heat, or a valve opening incorrectly (too early/late, far/little), etc. but that data from a measurement system on e.g. thermal or flow indicate the opposite, such as temperature doing down instead of up.

I wonder what the specs are for the systems they're using for telemetry... must be a gazillion channels at a high rate...

It means that the cause was surprising, not one that your intuition would think of.

It's sort of like "Prepare for unforeseen consequences."