Why does the kitchen water pipe bubble out as soon as the bathroom floor tile gap is opened?

According to your description, the cause should be a problem with the pipes under the ground, possibly a ruptured water pipe. It is recommended that you first find a professional plumber worker to test it with an instrument to determine whether it is a problem with the water pipe. If it is determined that the water pipe is the problem, then you need to knock out the tiles, then repair or replace the water pipes, and then redo all the waterproofing!

Maybe it's because the waterproofing of the bathroom is not done well.

There is a problem with the water pipe under the ground of your house, so find someone to repair it before it floods the downstairs, or the cost will not be a few hundred yuan to solve.