Discipline can become a way for divorced parents to continue expressing hostility with an ex-spouse. The children may be angry to begin with. They did not choose to have a stepparent.
They may not have a personality or interests that match the stepparent. The stepparent chooses a spouse. The children were part of the package. The stepparent may - even though it can feel wrong - not like the children.
In biological families, rules and methods of discipline evolve over time. It's much harder when remarriage demands instant rules. Different households may have very different sets of rules. Stepchildren over about 4 or 5 are likely to resent discipline by new adults no matter how fair.
Stepparents, trying to please both their new spouse, and their children, send mixed messages. Counselors unfamiliar with stepfamilies may add to stress by recommending disciplining children the same as in a biofamily.