Why did the Mempool suddenly drop to zero What caused the huge drop of the mempool from 60 MB to Less then 1 MB?

These transaction didn't get processed there was no 60 MB block mined at the time.

This occurred at 10:38 pm 29th August 2017 NZ time
It didn't goto zero. You can look at alternate sources https://core.jochen-hoenicke.de/queue/#2d . What happened most probably is that blockchain.info's node went down/rebooted momentarily.