This is about the Default Bearer, and even more about the signaling part of default bearer between UE and Radio.
As soon as eNB/eUTRAN notices the UE is not transmitting any data it switches its state from CONNECTED to IDLE.
But the eUTRAN also assures the EPC side that the UE is still connected.
The UE has the same IP address allocated on the initial Attach procedure. EPC is aware of existing bearers, they are just not kept on the radio/eUTRAN side.
When the traffic is sent/recived for this UE, the establishemtn procedure happens only on the Radio/eUTRAN side.
There are many timers on the Radio/eUTRAN side that could help you understand the above, but as I'm not Radio person, please look them up yourself.