The converter is stalling when it happens.
Because there is no direct connection in the torque converter, the impeller will always spin faster than the turbine – a factor known as "slippage." Slippage needs to be controlled, otherwise the vehicle might never move. That's where the stall speed comes in. Let's say a torque converter has a stall speed of 2,500 RPM. If the vehicle isn't moving by the time the engine (and therefore the impeller) reaches 2,500 RPM, one of two things will happen: either the vehicle will start to move, or the engine RPM will stop increasing. (If the vehicle won't move by the time the converter reaches the stall speed, either it's overloaded or the driver is holding it with the brakes.)