It's more of seeking for additional profit. Altcoin season usually happens once bitcoin is having a big rally. To maximize profit at time like this you need to rotate your profit from bitcoin to altcoin before it's going up.
It could be true to some extent but just because altcoin season happens in previous cycle doesn't mean it will happen right after bitcoin bullrun again.

It's more of a bet in my opinion where you're betting the bitcoin dominance to dump so sharp and altcoin price increase.