It´s this rule:
How Do You Retreat Using Double Energy Cards?
Paying Retreat Costs can get confusing with Double Energy cards. Here is the way it works: Discard Energy cards one at a time until you have paid the Retreat Cost (or maybe more). Once you have paid the cost, you cannot discard any more cards.
For example, suppose your Pokémon has a Retreat Cost of CC and it has 2 fire Energy cards and a CC Energy card attached. You can pay the Retreat Cost in several ways – by discarding CC, by discarding 2 fire, or by discarding fire first and then CC. You cannot discard all 3 cards, though.