Depending on where the bubble is on the tire, could be a couple of reasons.
The bubble could be from damage when installed. If they tore a small portion of the bead when mounting, this could allow air to get between the inner casing and the sidewall. Had this happen when I had a flat repaired. Lasted about a month and then all of a sudden had a small bubble on the outside. VERY large bubble on the inside of the tire.
There is a possibility of your hitting a curb and doing damage to the sidewall. This also will not show for a while.
Just a couple of possibilities.