This can happen if there is a loose connection, a broken wire, broken insulation, etc.
The usual symptom of these types of faults is you can measure full voltage when there is no load, but devices that you plug in just don't work (or are slow or dim). This happens because the small gap in the conductor caused by the loose connection or the broken wire is very resistive at high loads. Broken insulation can cause the same problem because the live conductor may be exposed to an adjacent metal conduit, ground conductor, etc.
However, depending on the nature of the break, the added load of one of these faults could trip a breaker.
Since you say that this circuit works fine when running your pool pump, I would inspect all of the wiring specific to this outlet. The short/break could also be inside the outlet.
You also mentioned "another breaker box outside". Does the outlet you're using have a breaker in this breaker box? If it does, I would investigate that breaker as potentially faulty. It may be that whatever device you're plugging in along with the pool equipment is drawing more than 30 amps and tripping the upstream breaker because the outside breaker has failed.
My final thought is that your pool equipment might be faulty and drawing more current than normal. The added load of your power tools could be the straw that breaks the camel's back.