Coincidentally, a week ago, I used a "self-leveling" mix on a bathroom floor. Concrete was used first to bring the floor up to within an inch of where we needed it to be. The next day, it appeared that all was well. The day AFTER that, I walked into the bathroom and saw a big bubble in the middle of it all. An air pocket... but why? Has anyone else encountered this? Probably the same reason as the concrete bubble?
G.P.R.

Use a poke vibrator to ensure all air pockets have been removed. The density of your concrete was probably too low use an Ad-mixture to get the same strength and workability at lower water: cement ratio.

This pour was very small... maybe 3/4" - 1". I didn't think a poke vibrator would be necessary for such a small job. By density being too low means there wasn't enough water in my concrete mix? Would you please clarify what you meant by too low? It wasn't dense enough?

When concrete cures its core temperature rises.. this would heat up water molecules in the concrete mix more rapidly than the aggregate. Water molecules would rise to the surface and evaporate causing bubbles in the concrete. You could use a retarder to increasing curing time thus lowering concrete curing temperature but this is a little extreme for a surface bed. I suggest using less water in your mix.