Why is that frost fan running during the day?

Frost fan operation must comply with the local district plan.
The Marlborough District Council has specific rules for frost fan operation.
Additionally, if they have required a resource consent to be installed they may have conditions that accompany that consent.

Now if you see a frost fan running during the day it could be for a few reasons. Some good some bad.

Good:
They are testing the fan prior to use for a season and ensuring it's safe mechanical state (= responsible running). OR
Bad:
They may be testing the fan prior to use for a season and the wind is blowing at the same time. This is BAD because frost fans can be dangerous to operate in windy conditions. (= bad and dangerous practice that endangers the workers testing the fans and neighbors within fan blade flinging range)

Good:
The fan may simply be free-wheeling in the wind. No problem there. The motor will not be running and the fan will not be turning at anything like its normal operating speed. (= Just fine. No problem. Even in wind)

Bad:
The vineyard operator may be trying a new trick that has recently been used in Marlborough where the fan is operated just prior to harvest in an attempt to dry the vines to control build up of fungus. This is a naughty for a couple of reasons. Firstly, it's a violation (in Marlborough) of the District plan. Secondly if the wind speed is anything above a crawl, it could be dangerous to operate. (= Naughty. Naughty. And neighbors noticing this should contact the Council to make a complaint)