Unifying two topics here.
A hard fork is in some ways akin to the creation of a new alt.  A difference being that it happening fairly rapidly where one of the chains typically survives at the expense of the other.

Rare events such as a hard fork are edge cases wherein unique opportunities for mischief present themselves.  Some of these are fairly obvious others are less obvious.  Certainly there are ways to amplify an economic attack through various types of DDoS as has been often done in the past.  These should be well and fully anticipated, especially by exchanges, and most especially by exchanges that may deal with both forks.