Drinking water/fluids will make a person urinate. Thats why in "shy bladder" situations, the collector has the donor sit down and drink water (within a 3-hour window)until the person can produce a sample.

And it did not "clean out" your bladder - it merely diluted (thinned out) the THCA in your bladder to where the sample registered below the cutoff thresholds of the drug screen. This action is only temporary - once the fluids taken in had all been urinated out, the "dirty" donor will resume testing positive for THC.