Are banks scared of some risks of money laundering and other illegal activities, or are they simply trying to hurt people who use cryptocurrencies to prevent mass adoption?
It's probably a mixture of everything, but it largely comes down to crypto forming a threat to their business structure. Another possibility is that they are looking to hammer down on everything because of their own plans.
Nowadays we more and more hear about how banks are interested in developing their own crypto 'trading desk', and this might be somewhat related to it. It could be seen as early elimination of competitors.
With how crypto exchanges gain more popularity, their fee income is growing as well, and that might be the underlying incentive for banks to cut off any connections from and to these exchanges. It definitely could be that.
It's an easy but yet very effective way to deal with competition. Just think about it, if there legally isn't a problem to let people and businesses use their bank accounts like that, there shouldn't be a reason to close them.