Problems with basement bathroom and floor drain

Hi all,
Over the past 12 months, I've been working away at putting in a new bathroom/spa in the basement for the wife. Everything is pretty much done and I did pay a plumber to come and do the drains for the bath, sink, shower, toilet and relocate the floor drain. I don't feel comfortable with touching anything related to the main drain.
He did all this work in January and I'm finally getting the bathrrom finished to the point of using it regularly. A couple of days after I put in the toilet (Toto Carlyle as recommended by the plumber), I had some water come back up through the floor drain now located in the adjacent room after the toilet was flushed. It looked like there was an air bubble of some kind coming back into the toilet at the same time. OK, that's a drag, so we checked the drain with a snake in case there was something in the pipes and I headed over to Home Depot and picked up a backflow preventer. I put it in and tested, but the problem didn't recur. Today, the wife is cleaning the tub and just running water in it and down the drain. She goes into the laundry room and notices a little bit of water on the floor. That's really weird, because we haven't had any issues related to water from the tub in the past (she has been using the tub off and on since May).
Anyone have any ideas on 1) what the air bubble might have been and 2) why this whole back up thing might be happening?
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