For incoming:
-Congestion
-radio problems (interference, HW faulty, bad parameters setting)
-Wrong neighbors relations definitions (intra BSC, Inter BSC, Inter MSC)
-Ghost HO (BCCH/BSIC conflict)
-BSS problems (SW and/or HW, Transmission)
For outgoing:
-target cell congested
-target cell is facing radio or BSS problems
-target cell has same BCCH/BSIC with non defined nearest cell
_wrong parameters setting in target cell
etc…