Why is 2-trichloroethanoic acid such a strong acid?

Cl is very electronegative, which makes it electron withdrawing. This means the three Cl substituents pull electrons from the O-H bond. This causes further polarisation of the O-H bond, which means there is a strong tendency for the H+ to be lost from the molecule. Furthermore, the Cl groups then can pull electron density from the negative O-, which will stabilise the ion. I.e. the negative charge experiences some delocalisation. This means there is a decreased likelihood the ion will just reattach to a lost proton (H+), causing there to be a large concentration of H+ within the solution. This gives the lower pH.