I'm making speculations, half-guesses and just plain guessing here. Be sure to rip it apart.
Point 1:One or both of the two lost legions fell to Chaos,prior to the heresy.
Point 2:The Space Wolves were involved in some way.
Point 3:The Eleventh apparently overstepped it's mark in some way, possibly while in a squabble with the Second. The Second possibly fell to Chaos.
Point 4:After all was said and done, the Emperor was unwilling to speak of these events.
Draw your own conclusions.