Automatic garage door opener opens when it hits the floor
This is most likely due to the auto reversing function.   This is where the door will reverse open if it hits an obstacle when closing.

One possible cause is that there is an obstruction on the floor where the door meets the floor.  Look for any irregularities on the floor and on the bottom of the garage door.  You might find something stuck to the bottom garage door seal. 

Another possible cause is that the door got out of adjustment.  Most door I have seen have four adjustments; 1) Open force; 2) close force; 3) open travel distance; and 4) close travel distance.  I would look at making adjustments to the close travel distance.