I attempted to edit and add the following comment, but apparently the edit button has been excluded.
If the WC is of the "water saver" type and you hold the flush lever down, thus initiating a double flush, that vacating of fluids in the mouth of the trap leaves an air space, causing bubbles.
Other may as well be a configuration of the rapidity of the expulsion.
Nothing to worry about, just resist the temptation to gasp in amazement.