Consider:
If you don't use the proper technique there is a high probability of injury and it happens very often.
If you don't use the proper technique there is a high probability of injury and that happens very often.
Neither sentence is very good because probability doesn't "happen". Injury happens.
It would be better to say:
If you don't use the proper technique you can easily injure yourself.
Now we can choose between it and that :
If you don't use the proper technique you can easily injure yourself—and it does happen fairly often.
If you don't use the proper technique you can injure easily yourself—and that does happen fairly often.
Since you're pointing back at what you have just said, use that. That points at the thing. It is simply a substitute for the thing. Furthermore, that in such a context always points back at what has just been said. It, on the other hand, can also anticipate or look forward to its reference. If you use it, the listener might think you're about to say something like this:
If you don't use the proper technique you can easily injure yourself—and it does happen fairly often that lifters injure themselves doing this lift.
For these reasons, that edges out it here on the basis of clarity.