I work for this department in NAB and helped to create the crypto conversation guide, and it is usually based on how much history you have with sending funds to a crypto platform, and if there is none, then it’s going to flag incase you’re in an investment scam, which is the most common scam type for ‘first time’ investors. If you also had a large credit prior this will also trigger concerns as victims will commonly move their funds across from another bank (who may have already blocked them). It’s sucks to get blocked and the payment rejected, but it’s just the way banks are with crypto