Dismemberments work not only by sudden impacts, but they also take into account the pressure applied to the joint and the normal amplitude of movements of said joints.
The factors explain pretty much every dm, including wrist dms.
Wrists are just naturally more fragile, and the fact that they can engage in solid grips, restringing their movement and preventing them to adapt to the impacts they suffer, makes them even more susceptible to dms, that's why they sometimes seem "random".
You also need to remember that wrist contraction is more powerful than wrist extension.
So my best advice is: if you're trying to dm someone's wrist, just force it to make a movement it can't do. For example, make his wrist over-extend, or kick it into his arm, making the wrist cave in and dm.
On the other way around, if someone tries to over-extend your wrist with theirs you can counter it by contracting your wrist, since here in most cases contraction has more force than extension. If they try to break your wrist with a kick and you intend to block it by grabbing the kick, you need to make sure that your arm is mobile enough to be able to absorve the impact (a fully extended arm is too stiff and will easily break with enough pressure).