Fortunately, Chase will sometimes review the decision and overturn it. When people ask Chase why they were shut down, Chase often refers to the large number of accounts they opened (not just with Chase) in the past year. Sometimes it seems to be triggered by an abrupt increase in spend overall. Sometimes it happens after making a single very large purchase.
This often happens soon after opening a new Chase card. Reports continue to trickle in from people who report that Chase closed down all of their accounts.