It is their own laws they are breaking, the legal framework they set and required to become a member of the EU. This violation of the requirements they set led to the Euro Crisis and then put financial and political pressure on the UK to leave the EU. The whole point of having treaties is to meet the agreements set, when these agreements are broken the contract is in breach and compensation is made or the contract is dissolved.
A better question to ask is was there a Euro Crisis? Why? Because the ECB, the ESS and the EC failed to implement the targets they were legally bound to achieve. If the EC does not seek the best possible trade deal with the UK it will be a failure in stewardship of the best interests of economic governance, which they are supposed to be providing. It would put the EC's actions outside of its own legal requirements.