Side bulging is formed on the side wall of the tyres, side wall is the part of the tyre where all the information about the tyre is written like tyre size, company name and model etc.
It is formed due to sudden impact of tyre on a rough road or the tyre being impacted strongly by a rock or a pothole which can happen on driving on high speeds or rash driving and not slowing down on un levelled roads or potholes. The impact on the outside causes the tyre belts on the inside to crack up which causes the layer to open up and air get through that layer and causes that particular impacted area to get swollen up or as you may call it, sidewall bulge or sidewall bubble
*The first image is the representation of the side bulged tyre on the outside and the second picture shows the damage happened on the inside of the tyre*
There is no particular solution to sidewall bulging except getting your tyres changed as soon as possible. It is strongly suggested to not drive on a side bulged tyre because it can burst at any moment