Connection object should be deleted to allow dropping of collection.

[egorchatsperasoft]

CONN = Connection(username="root", password="...")
DB = CONN["DB"]
DB.dropAllCollections()

doe snot really drop collection, until CONN object exists

if you write again

CONN = Connection(username="root", password="...")
DB = CONN["DB"]

you found them dropped

if you even write

CONN.resetConnection(username="root", password="...")
DB = CONN["DB"]

collection will survive.