Why is this incremental model reprocessing everything?

A dbt incremental model passes every test but its runtime grows every day, and the warehouse bill with it.

sql
{{ config(materialized='incremental', unique_key='order_id') }}

select * from {{ ref('stg_orders') }}
{% if is_incremental() %}
  where updated_at > (select max(updated_at) from {{ this }})
{% endif %}

Give two plausible causes.