You might want to google “choked flow†(or "critical flow") and do some reading on that. Choked flow occurs when the flow through a restricted area, such as an orifice or nozzle, can no longer be increased by a reduction in the downstream pressure. This occurs when the sonic velocity is reached at some point along the flow path.
Say, for example, you have a high-pressure air line with a small valve on the end. When the valve is opened, air will rush out the opening until the air flow reaches sonic velocity, at which time the flow will become choked and there will be no further increase in velocity. If the flow is choked, further lowering the downstream pressure has no effect on the gas flow. Although the gas has a velocity limit, a higher mass flow rate can be attained by increasing the upstream pressure, which increases the density of the gas as it flows through the opening.
A De Laval nozzle is an example of choked flow. We have a very high upstream pressure in the combustion chamber and a very low downstream pressure at the nozzle exit. The flow as it passes through the throat will become choked, and therefore, will be at sonic velocity.
I don’t understand the dynamics enough to explain why there is a sonic velocity limitation -- there just is. It probably has something to do with pressure waves and the like. It is not my area of expertise.
EDIT:
Why doesn't this happen if you don't reach the speed of sound in the constriction?
Anytime the pressure difference across the throat exceeds a minimum value, the flow will become choked. A rocket engine far exceeds the minimum value, thus the flow is always choked and the velocity at the throat is always sonic velocity. I believe that for air, the flow will become choked when the upstream pressure is about twice the downstream pressure. In a rocket engine, the upstream pressure can be hundreds of times greater than the downstream pressure. The divergent part of the nozzle will accelerate the exhaust gas whether the flow is choked at the throat or not, but for the flow not to be choked, the pressure in the combustion chamber would be so low that we would have a very non-effective rocket engine.