How can advair and symbicort (budesonide and formoterol) increase the risk of asthma related death?

See below: Long acting beta agonists (labas) are not to be used for except in patients where their asthma has not been adequately controlled with an inhaled steroid. It increases the risk of death otherwise per reports. Why? There are many reasons which could include: mistaking using the med during an acute asthma attack, hypersensitivity to either of the drugs or milk products, cardiac arrhythmia, & other.