There are a few reasons why Shopify might automatically fulfill your orders. The first reason is that you have the "auto-fulfill" setting enabled in your Shopify admin. This setting can be found in your Settings > General > Store fulfillments section.
Another reason why Shopify might automatically fulfill your orders is if you're using a fulfillment service like Shipstation. This integration will automatically send your orders to Shipstation, which will then fulfill the orders for you.
The last reason why Shopify might automatically fulfill your orders is if you're using a Shopify POS system. When you make a sale using Shopify POS, the order is automatically sent to Shopify and fulfilled.
If you're not sure why your orders are being automatically fulfilled, you can always contact Shopify support for more information.