Board Thread:General Discussion/@comment-27596920-20141205004927/@comment-25555436-20141206200738

To know how she died, we need to know where. The areas outside the Kingdoms hold old, dangerous Grimm, some experienced enough to pose a threat to even a group of experienced hunters. It's possible she was sent on a mission there, for X reason, and she died there.

If she didn't die outside one of the kingdoms, it means someone killed her. However, who could have done so, and why, I don't know. The weaker possibility is that a less powerful Grimm killed her, but to kill an experienced hunter like Summer...something must have happened.

One thing to note is that she went MIA a few years after Raven. Maybe the cause is related.