D&D 5E As DM, Do you have monsters always fight?

[Seramus]

That could mean they randomly wander into each other, but it could also mean that the PCs happen to pass by the trolls’ camp and smell the mutton they’re cooking.

Same. I look at what RNG gives me and try to improv the best story I can out of it. It's more likely to be a trolls-around-mutton situation than a straight fight. And when fights do happen, the enemy flees when reasonable to do so.

Consequently, this does mean I tend to lean on Deadly encounters.