Car accident

The car accident was a major event the occurred long before the creation of Kumon-mi, while Sensei was still in college. It resulted in the deaths of Sensei's brother and his brother's wife, and depending on the interpretation may have also resulted in Maya Prime's coma.

Early car accident
In one theory, the car accident occurred earlier in the timeline, when Maya was only seven years old. Sensei Prime was likely in the car as well, although walked away largely unscathed and saw his brother's wife mangled in the wreckage. This accident resulted in Sensei Prime becoming Maya's caretaker until their eventual relationship the resulting incidents.

Due to the necessary age differences involved as well as the parallels to Ami's backstory, this is the more likely theory.

Late car accident
An alternate version of events entails the relationship between Sensei and Maya occurring first in the time line, while her parents were still alive. At some point after the abortion, the accident occurred with all four of them in the car. Sensei's brother and sister-in-law were killed and Maya fell into a coma in this version of events, with Sensei again surviving to go on to create the Upside Down House.

Within Kumon-mi
The emotional trauma of the car accident and the resulting fallout were so great that it couldn't be fully wiped from Sensei's mind even inside the fictional world of Kumon-mi. As a result the event was retconned to serve as Ami's backstory.