Any number of reasons why they might need to do it now:
*Financial Year end of 31st July, and today was the day the auditor could come out and witness / sample check the stock take
*Arranging finance with the stock as security – provider needs to verify that the stock exists
*Arranging finance generally, with stock being a major component of their balance sheet the provider needs to be assured that it is as it is said to be
*Looking to sell the business and an agreed value for stock is being sought.
*They know or suspect there’s been some “wastage” (theft) and need to know what stock they actually have as opposed to what the computer says is there.
*They’re about to do an end-of-season sale and need accurate stock figures to decide what to discount, and by how much.
Or, they just decided to pee you off and do it today to inconvenience you.