Tilt a table with a book on it, slowly: for a while nothing happens, then at one particular angle the book gives way all at once.

The ramp pushes back less#

On a level table the normal force equals the weight. On a ramp it balances only the part of the weight across the surface, $mg\cos\theta$. The other part, $mg\sin\theta$, points down the slope and tries to slide the block.

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When the block gives way#

Static friction is a response: it takes whatever value stops the sliding, up to a ceiling. The block slides when the pull down the slope passes that ceiling:

$$\frc{f_s} \leq \mu_s\,\frc{N}, \qquad \tan\theta_c = \mu_s$$

Why it lurches#

Once the block slides, friction stops adjusting and locks to a fixed value, set by the kinetic coefficient:

$$f_k = \mu_k N$$

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What comes next#

You found that a ramp presses less as it steepens, that grip gives way at an angle no mass can change, and that sliding friction is the weaker kind. Next the force turns perpendicular to the velocity: the speed never changes, and the motion never stops turning.