What would cause a police office to turn on his lights when he is driving the opposite direction of you?

There might have been an incident calling for police on their car radio. They then put on their lights to attend and then perhaps another car closer to the incident said they were dealing so the car you saw was not needed.

There are a number of possibilities. MOST of them have absolutely nothing to do with you.

They may have received a call, then determine lights were not required.

They may have activated the lights unintentionally.

They may have intended to alert you are something.

It is even possible that you misinterpreted a reflection of sunlight off the lightbar as using the lights.

It could be to warn you of a danger up ahead or it could be just a coincidence that you were passing / there are cameras there as well so it could have been that he wanted to take a picture or like the other answer declared he could have just accidentally pressed a wrong switch or button

The first thing people think when they see a cop, is "oh, they might think I'm up to no good or they came for me". If the police wants to talk to you, they will. Don't assume cops are there to arrest every single person or give them a ticket.

to alert you to their presents.

to answer a call. then determined it was not needed.

to brighten your day, unlikely.

hit the wrong button.