Last minute mites population explosion

Hey Everyone,
I have a friend who started off their grow in a virgin facility, everything was brand new and had never had a plant inside their grow room. Got clones from some “expert” in Mi and of course they had mites and thrips. They managed to keep things under control with soap sprays and biologicals until the end of the crop with 2 weeks left in flower they stopped spraying and flushed the plants. The spider mites exploded, I have rarely seen such an increases in over 35 years of commercial growing. They gassed with Co2 4 times with limited impact and ended up vacuuming the buds with a shop vac at harvest.
Needless to say, they were disappointed in their first grow.
My question is this.
Is it typical to see a pest population explosion in the last few weeks of production ?
Granted they had stopped spraying, but is there a relationship between the pest dynamics and the plants becoming more favorable for their development as a result of the nutritional deficiencies caused by the flushing process.
And more importantly how can they prevent this on the next round.