Actually, the massacre wasn’t happening on Tiananmen Square, but next to it.
That one's true. There's probably been some deaths on the square itself but the bulk was in side-streets blocked by Peking locals in solidarity with the students, once the army fought itself through that and arrived at the square it issued an ultimatum and the students caved. Eleven of the withdrawing students were later killed away from the square by a stray tank crew. The tank man picture was taken the day after.
Don't get into fights with tankies over such things. Rather ask why the CCP was stomping Dengists -- which is what the students were.