Before getting into Cell Activation procedure let me tell when deactivation procedure will happen.
If eNB is serving more than one cell ,for Energy saving purpose based on cell condition(i.e presence of UE's),
eNB will deactivate that cell and inform to neighbors about this through cell deactivation procedure.
Now lets assume neighbor eNB say NeNB ,the UE's attached to the eNB are moving out of current cell border and towards the cell which one is deactivated because of Energy saving and no other cell are present ,in that case NeNB will use Cell activation procedure to activate the cell.
this is my understanding ,please let me know if i am wrong.