Originally the water for my humidifier was taken by flexible copper tube
connected to self piercing valve. The valve was installed below the heater
cold water shut off valve (the shut off valve shut water both for heater
and for humidifier). I recently had to move heater and re-solder some pipes.
I then reconnected humidifier tube and self-piercing valve above the heater
shut off valve. I fully opened the self-piercing valve for humidifier. Now
humidifier started to dip water, after night I found a lake under the
humidifier as if it gets too much water. Why this happens?

sounds like a coincidence to me unless something got disturbed in the
humidifier. You may have dislodged a piece of scale from inside the pipe that
has rendered your inlet valve of the humifier imoperable. YOur placement of
the piercing valve would have nothing to do with the leak unless water is
following the tube down below the humidifier making you think that the leak is
there and not up at the connection at the copper. Ck it out carefully to find
exactly where the water is coming from. If it is the inlet valve you may be
able to clean it.