A similar effect happened to me, where I was using and still use an extension cord to power my Kapex. The extension cord (2) had a small light in the end to indicate live power. Because I had two of those the two lights was enough to out from no where trigger the iVac. I removed one of the extension cord and it never happen again.
Your case is a little bit different. Could it be because your outlet used to power the tool is on the same circuit you were using the sander? This may occur if both outlet are on the same breaker. Now if they are on separate breaker, the only thing I can see is a ground fault. If you have access to an ampmeter

clip it on your iVac outlet when powering and check if you get any reading before and after using the sander from a different outlet.