Hi,
Of course the drop can also happen due to some BTS issue, pre-emption, integrity check failure of some RRC message during e.g. radio bearer reconfiguration procedure (which happens failure often during UMTS nrt data call) & release from PS core (e.g. mismatch in Tias/Tiar timers between Core & RNC). I have also seen some UEs, which are using RRC:signallingConnectionReleaseIndication for PS CN wrongly and this is seen as RAB drop by some vendors’ RNC.