That's kind of bizzare to resign shortly after winning reelection, before your next term even began.
It's not at all unusual in IL. If a resignation is likely (as it was here) then the organization benefits by waiting until after the election. There is always the risk of an unknown winning if the seat opens up or appears to effectively open before the general (see for example the indicted Rostenkowski in 1994). With a special election organizations can better get their voters to turnout for the replacement. For lower offices in IL there is no special election and by waiting until after the election the party in power has the ability to appoint a successor until the next election.
I don't know about this, but it's clear Lipinski's father resigned after the primary just so the machine could pick his son. Someone really needs to primary Lipinski.
