Why is my defense line broken when I log into game yet I have aegis up and no trace of attack?

There is a chance that your corps unleashed the "Turkey Event" which is a corp-specific event where an enemy AI attacks the bases of everyone in your corps regardless of being bubbled or not... including yourself.

Fret not, you can always check the battle report too to find out who hit you. Chances are they were turkies xD. The good news is if it was an attack from turkies, you only get wounded troops. Better communication with your corps can help alleviate this issue.