ignore the conspiracy folk.
its not random but driven by the various bank algothithms. each bank will have their own, so starling will be different from rbs from lloyds to santander etc. as the recipient, you wont be told it was blocked.
generally if it was you making the payment. it may be your turn, it may be your spending pattern - too much, outside the usual category etc, it may be the recipient and so on. what the bank employee sees will be dependent on the software used. ie if its card transaction it can usually tell if the pin was used, card not present, phone transaction, if through online banking did the person sign on with the normal ip address, how they logged on etc
the security questions can often be the hardest even for the genuine customer to get through - card limit, last few transactions etc
I did it for a few years and my attitude after a few days was basically why is this fecking person fecking lying to me when trying to claim someone had stolen their card and used it to make a cash withdrawal using the pin but then putting the card back in the wallet without them noticing. you quickly had a gut feeling who was genuine and who wasnt.
yelling at me saying you had a train or bus to catch usually resulted in slow system or a bad line 😈
problem is that quite a few times the anti fraud text messages asking people to confirm if something was real and could be released automatically, were thought of as fraud by customers and ignored. so no reply was received and the transaction was blocked!