How can one be arrested when case is nolled?

Technically, one cannot be arrested when a case is nol prossed (nolled, nollied). Actually, though, one COULD be arrested again, if the proper paperwork has not arrived where it should (the clerk's office, the sheriff's office, the probation office, etc.).

If a new case has been filed on the same incident, one can be arrested again.

Additional: The final sentence of the first answer is absolutely correct.

Just because the prosecutor declines to prosecute the offense you were charged with (Nolle Prosequi), does not make it 'go away.' A Nolle is NOT a court dismissal of the case, and in the event subsequent information or evidence comes to light, you can be re-arrested on that same charge. This is NOT double jeapordy.