X2 handover can be triggered only if x2 interface is available between source eNB and target eNB. There is no relation of A2 with X2. X2 interface just make handover faster as compared to s1 handover. Event A2 is triggered when serving cell becomes worse than configured threshold. RSRP or RSRQ is being for threshold. eNodeB also configured number of report need to be sent from UE to eNodeB.
Please check 36.331 for more details.