I imagine it's the combination of evolutionary pressures of being a scavenger and having to fly. Since crows tend to steal food from dangerous animals like wolves, they need to be able to react quickly and to be able to predict behavior of other animals. This selects for intelligence and a theory of mind. Meanwhile, flying means you have to keep things lean.

It's also worth noting that the human brain has massive redundancy in it. For example, there was a case of a guy in France who was missing 90% of the brain due to an extreme case of hydrocephalus. He didn’t even realize there was anything wrong with him until he went to the doctor for some unrelated problem, only to be referred to a specialist because his head seemed a bit too large.

https://www.cbc.ca/radio/asithappens/as-it-happens-thursday-edition-1.3679117/scientists-research-man-missing-90-of-his-brain-who-leads-a-normal-life-1.3679125

In light of that, crows having high levels of intelligence doesn't seem that weird at all.