In some networks T300 value is high.
Signaling phase somehow pass the RRC phase but unable to pass the ERAB Phase.
For example when a mobile try to attach they pass the RRC phase but fail the ERAB Phase due to say poor coverage.
Normally the MME timers are larger than UE timers. UE timer expire and UE re-initiate the attach procedure.
When UE reach the ERAB setup phase MME came to know that context of same UE is already present.
So MME discard the old request/context and treat the new one.
This type of failures pegged in MME releases.
It was due to one or few UEs in bad coverage but they degrade the ERAB setup success rate at cell level.
If you have CTR please do check the same. I found many such cases in Huawei.
It’s due to whether ERAB message size is large or anything related with the robustness, I am not sure.
But this phenomenon exists. Sometimes changing weather like rains, etc. Or change in indoor traffic profile e.g. lock down due to Covid also adds to the misery.
Pass or Establish. Whatever word seems appropriate to you
If your vendor is Ericsson do check UL SINR, or Poor Coverage counters as well.
Final Addition: UE timer expires while MME timer is still running.
UE re-initiate the attach procedure and during this when UE reach the ERAB setup phase MME know that context of this UE is already present.
So MME discard the old request and treat the new one.