Quite a few Shopify themes redirect to the cart after a user has added something. This is becoming less common now as most merchants are opting for no redirect.
This is because a customer will tend to purchase more in a single order if they aren’t taken straight to the cart. It gives you a chance to upsell as well and increase your Average order value.
If you have setup an Action Upsell but your theme redirects to the cart without a chance to show it, you simply need to disable this.
Most themes have a setting on their Product Pages in the “Customizer” in Shopify to do so.
Some are a bit more tricky. For example, if you use “Debut” or a similar free Shopify theme, you will need to edit a few theme files. If you aren’t comfortable doing this, we can do it for you.