Hi Pin...
Here goes my 2 cents:
EECs needs to monitor a variety of parameters to do his magic, right ?
Unfortunately, some of the information used by the EECs is delivered by the DEUs.
A DSPLY SOURCE indicates a DEU problem... This way, EECs will lack some of the information needed and will initially revert to SOFT ALTERNATE (using the last valid parameters as reference).
Hope that helps !