My guess is that the fact that the halving wasn't priced in the previous two times is decent circumstantial evidence it isn't priced in this time as well.

To me, the reason it goes up is simply supply/demand economics.

You have a balance of buyers and sellers to keep prices relatively flat prior to the halving. The sellers include the new emissions with each block.

The current demand levels roughly offset those sellers in order to keep prices relatively flat.

If you remove half of the new emission rate, yet the demand stays flat, goes up, or simply doesn't go down as much as the emission rate decrease, the price has to go up.

When the price starts going up, traders and speculators jump in and prices end up going higher than they should.

Poof, post halving euphoric rally!

As it relates to altcoins...

First bitcoin then the altcoins.

Gains made in bitcoin will eventually want to be diversified out into other assets. In some countries moving from bitcoin to other cryptocurrencies is not a taxable event. Plus you have retail getting into crypto due to the excitement bitcoin is causing as it makes new high after new high and they tend to want to catch "the next bitcoin"... IE the small, cheap, altcoin that has more upside potential.

Poof, altseason is here!