Is it normal for hot mustard oil to foam strongly when ingredients are added, and why does it?

Foam is caused because of the chemicals used. When companies use solvent extraction to extract the last drop of oil from seed, it does leave a trace of chemical which is left behind even after refining to filter out those harmful chemicals. Also they extract oil through high temperature mechanical extraction process prior to solvent extraction. This means the final oil that comes out has a shorter shelf life, so companies use preservatives to increase the same. This also plays a roll in forming those foams.

This is because all the Indian brands I know use the cold press method. In that process, some moisture is left in the oil that makes foam while heating. It's absolutely normal, and it's good that it has no chemicals. Hope this will help.