The physics
Push a crate gently and it stays still. Static friction pushes back just as hard as you push. It grows as your push grows.
Static friction has a limit, μsN. On a flat floor, N = mg. When the push reaches the limit, the crate breaks free.
crate stays: f = P f ≤ μs N it starts at P = μs m g
Once the crate slides, friction drops to μkN. μk is less than μs. So friction jumps down just as the crate slips.
The limit depends only on μs and N. A load on top raises N, so the limit grows. The contact area does not matter.
The floor's two pushes, N and f, add head to tail. They make one contact force R. At the edge of slipping, R tilts the most it can. That tilt from the vertical is the angle of friction.
R = √(N² + f²) tan θ = f / N = μs at the edge