Unusual large quantity orders – innocent or fraudulent?

I’ve been selling on FBA for the past couple of months. The buying pattern on the product is pretty consistent with buyers usually ordering 1, sometimes 2 products per purchase – which makes sense for the product. In the last few days I’m seeing larger and larger quantities of orders of 3-5 units per order, with a really strange one coming in today with 18 units in one order (I didn’t have a max amount limit).

18 units – what are the chances this is an innocent purchase?

What should I keep an eye out for? What is the buyer’s intention?

Thanks!!