People used SVB because (1) It was a normal, trustworthy bank with many years of regular operations, (2) It was one of the few banks that would work with early stage startups that have lots of funding, (3) in some cases they were basically forced to by the VCs that funded them and (4) SVB had a well-earned reputation (and now well-lost).
The collapse of SVB was not made by any group. It was made by a relatively small number of individuals who made bad investment decisions. These is no reason to believe that they bought MBSs because of groupthink.