Tiny Tina's Wonderlands random encounters are triggered just like in Pokemon

gamesradar.com:

Tiny Tina's Wonderlands is all about those tabletop RPG vibes, but the random encounters will take you straight back to Pokemon Red and Blue - if you want them to, anyway.

The travels you'll undertake in your Tiny Tina's Wonderlands campaign are represented by a miniature-scaled overworld map, which you can either drill straight through in an implacable quest for the end credits or explore to find secrets and side quests. As revealed in a new IGN preview, the overworld map is also sprinkled with patches of tall grass. Stop us if you've heard this one before, but if you walk around in the tall grass? Yeah, you're gonna need to fight some dudes.

Don't worry, it doesn't go all the way into retro RPG territory by locking you into turn-based attacks with your opponents. Instead, you'll drop into a small, dedicated arena map where you can battle it out with whatever fools are trying to crash your adventuring party. Then you can reap the XP and loot rewards, and if you're still feeling under equipped for your next quest, you can head back into the tall grass to do it again, and again, and again.

As for what kinds of characters you'll be leveling, a Tiny Tina's Wonderlands trailer recently went in-depth on two examples: the Graveborn and the Spore Warden. You'll be able to start power leveling them and all the rest when Tiny Tina's Wonderlands hits PC, PS4, PS5, Xbox One, and Xbox Series X and S on March 25.

For more on Gearbox's next title, Play Magazine's Tiny Tina's Wonderlands preview from January found it to be a smart distillation of the core Borderlands experience.