Not routine
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It's not routine. He perhaps felt that something was going on and wanted to further investigate for his safety.
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I wouldn't say it's routine. But if the cop had any reason to believe you were under the influence, or that you being in your car posed a threat to him, then he can pull you out.
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He could have felt threatened. You could have matched the description of someone. He could have found something when he ran your plate.

A lot of could haves.
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Its whatever the policeman feels it should be. If they feel there is a reason to suspect you of something they absolutely can ask you out of the car and you don't have any case.

Had there been a problem you would have been arrested so I would just move on.
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Not routine I would file a complaint otherwise the jerk will think he can do it again
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No. Just the driver's license. You get to stay in your car.
Unless he suspect there is something unusual going on other than the traffic violation.
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HE can ask you out of the car if he wants, he can pat you down if he has reason to believe you might have a weapon on you, and if, he is a state trooper (in NJ) he can arrest you for any crime, regardless of how minor
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Not routine but I'd suspect you'd have the right to ask questions and be suspicious about sitting in his car. We've had issues here with people faking like they were cops. I know I wouldn't want to get into a cops car unless he had a reason to arrest me. I'll sit on the ground please.
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"[I]t is simply fantastic to urge that a ['stop & frisk'] procedure performed in public by a policeman while the citizen stands helpless, perhaps facing a wall with his hands raised, is a ‘petty indignity.' It is a serious intrusion upon the sanctity of the person, which may inflict great indignity and arouse strong resentment, and it is not to be undertaken lightly."

- Terry v. Ohio, 392 U.S. 1,16-17 (1968)

"The frisk . . . was essential to the proper performance of the officer's investigatory duties, for without it, ‘the answer to the police officer may be a bullet.'"

- Terry v. Ohio, 392 U.S. 1, 8 (1968)
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Over thirty years have passed since the United States Supreme Court's landmark decision in Terry v. Ohio, 392 U.S. 1 (1968). In Terry, the Supreme Court upheld as constitutionally permissible the police practice of temporarily detaining civilians for investigatory "stops" and, under limited circumstances, subjecting them to protective, "patdown" frisks.

Being asked out of the car is sometimes routine, depending on the officer,department, etc. The patting down however I don't believe is routine. I've been asked out of the car once (15 years of driving) and was asked to sit in the front of the trooper's car, but there was no pat-down.
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It's not routine. Usually the officer will only ask you to stay in the car unless he suspects that you are driving under the influence and wants a closer look or a Field Sobriety Test.

Once you step out of the car, he needs your permission or a warrant to search the car for anyrhing he can't see through the windows.

The only thing I can think of is that there was an alert for a vehicle matching the description of your car, or a person matching your description and you were frisked for weapons while they ran your license for Wants and Warrants.
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Verymuch unethical on the part of Officer. At best he could have imposed you minor fine, so, that could have saved your time, also your offence could have compounded, then & there .The officer also could have engaged himself at the site, for any measure cause of the state.
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my personal opinion as a lawyer