Fairly common with most magazines in pistols and rifles, the spring pressure increases with each round loaded, eventually it will be high enough that the force to move the round down to lock the mag will exceed that provided by the weight of the weapon.
The higher the cyclic rate, the higher the base spring force must be, and the scorpion is quite high. At 30 rounds, the force is MUCH higher, I never load my mags past 25 anymore. If you set the mag on a hard surface and press down on the gun it will lock.