I don't know what you've found. Please share with us. I'm sure TFA protonates carbonyl group in CO(OBu-t) and the positive charge moves to t_Bu group that among alkyls stabilizes the positive charge the best. t_Bu+ cleaved from the molecule and gets quenched by any nucleophile available around, even if it'd CF3COO-. Obviously, CO2 released at the next stage.