The decrease in spawn timers is because of night time. Monsters will spawn near instantly at night, and keep harassing you forever. More if you're doing noisy activities like chopping wood and building.

Creatures in valheim aren't dumb, if you make noise they WILL come to greet you with a pointy appendage/stick/weapon.

Flat open spaces also lend themselves to this, low height mountains and plains often spawn creatures nearly instantly as you kill them.