Does anyone know what caused P0401?

Hi all,
Here's the scoop. This past Sunday I lent my 97 GV 3.3 and a 5' X 8' trailer to my brother to get some firewood. Before then all was good. I get it back, SEL is on. I get code P0401 "EGR Flow Insufficient" I clear codes, drove it for almost 100 miles so far and it has yet to come back.

Does anyone know what triggered the SEL? Maybe it's partially clogged and the load could have been a contributing factor somehow? Maybe under load the engine didn't produce enough vacuum when it needed to operate the valve?? I'm not looking to pin this on my brother, I'm just curious.

If it comes back on I'll definitely start with cleaning the junk out of it and the tube.

Thanks.

All the above. EGR is infamous for triggering intermittent codes.

For when the time comes: http://www.obd-codes.com/p0401

Thanks, RIP. I'm sure it will eventually pop up again, I'll be ready.