Translation:What is the real cause of this incident?
Can you explain the i on sebebi? If it's not a compound noun phrase, then it's in the accusative case? (My brain's settings, from Indo-European languages, won't permit the accusative case when it's being used with some form of "is." If it "is," then it's not being acted on.)
X-(n)-in Y-(s)-i = the Y of the X
olay-ın sebep-i = the "sebep" of the "olay" --> the reason of the incident
The -i is the 3rd person possessive suffix (its/his/her). The accusative suffix happens to have the same form, but you were right in saying that it can't be the accusative case here. :)
You're right, this is called consonant mutation. Another example would be kitap -> kitabı or etmek -> ediyorum
Please have a look here: