Not used to my newfangled Honda, why does it do this?

I've always owned manual transmission cars and just recently due to age and some other factors traded my last one in for a Honda. It's automatic obviously. My eyes still automatically look at the tach all the time though and I've noticed that when I go to pass someone on the freeway, it will rev up to about 2500rpms and then go back down. As if it is downshifting. Why does it do this?

You floor the accelerator the system assumes you want to accelerate - so it downshifts to get better acceleration.

I'm wondering if something is wrong with the car or if this is some kind of overdrive computer mechanism, haha, I'm very out of date on this.

It IS downshifting dumbass. It downshifts and upshifts automatically, which is why it's called an automatic transmission. The first A.T. Went into production in 1939, so how could you not know this?

If you have to ask this, you have NEVER driven a manual transmission. Only people that can only drive an automatic don't know that when the vehicle is shifted to a higher gear, the rpms drop. When you accelerate to pass, the rpms go up, until the transmission shifts to the next gear, and they drop back down.

It feels like it's down shifting, because it IS.

Modern cars are very high geared, with probably the last 2 gears actually being an "overdrive", to help the economy during highway cruising. But this means the engine revs are very low, and so producing very little power., When you give the car a bit more gas the computer knows you are asking for more power, so it drops down a gear or 2 to allow the engine to rev a bit higher and make more power.

This is one of the reasons what new automatic transmissions are almost as good as manuals for economy, and they are more popular. If you have a manual car with similar gearing you have to down shift (manually) every time you want to accelerate or come to a slight hill. With the modern auto it quietly drops a gear ratio and you barely notice.