Agree, standards seems to not clearly indicate when does MMEi decides to send detach request with IMSI detach. some reference below in case it helps ..
3GPPi 24.301 reads followings >>
When receiving the DETACH REQUEST message and the Detach type IE indicates "IMSI detach", the UE shall not deactivate the EPSi bearer context(s) including the default EPS bearer context. The UE shall set the MMi update status to U2 NOT UPDATED. A UE may send a DETACH ACCEPT message to the network, and shall re-attach to non-EPS services by performing the combined tracking area updating procedure according to subclause 220.127.116.11, sending a TRACKING AREA UPDATE REQUEST message with EPS update type IE indicating "combined TA/LA updating with IMSI attach".
According to 29.118, If the MME decides to perform a network initiated detach procedure and the state of the SGs association to a VLR for that MS is not SGs-NULL, the MME shall send an SGsAP-EPS-DETACH-INDICATION message to the VLR indicating "MME initiated IMSI detach from EPS services".