Was yelled at in Dambulla Cave Temple. What did I do wrong?

I am sorry you had a stressful experience, but this is a very strict cultural rule in Sri Lanka that often catches tourists off guard.

The Rule: You must never turn your back to a Buddha statue. Posing for a photo with your back to the Buddha (like a selfie or a standard tourist shot) is considered deeply disrespectful.

How to Photograph: You can photograph the statue itself, or stand sideways/at an angle looking at the statue. Never align yourself so your buttocks are facing the statue.

Clothing Check: Also, remember that "knees covered" is non-negotiable. If you have a sarong in your bag, always wrap it over your pants just in case the guards feel your pants are too tight (like yoga pants), which can also be an issue.