why the chemical structure of a carboxyl group causes it to act as an acid (release H+ into solution).?

The C=O of the carboxyl group is very electron withdrawing. So, electrons are withdrawn from the -C-O-H making the -O-H bond more weaker. Additionally, once the H+ ionizes, the carboxylate anion is resonance stabilized, also contributing to the acidity of the carboxyl group.

Carboxyl Group Structure