It's likely to be the drug as it is seen with all brands of injectable methylprednisolone. Btw, it happens also with similar MP pulse therapy in other indications.

In the literature we can find same case reports from different field of medicine describing bradycardia due to methylprednisolone therapy. Same mechanisms by which corticosteroids might produce cardiovascular side-effect have been proposed. Some authors suggest that methylprednisolon might have a direct effect on the integrity of cardiac muscles, causing alterations in Ca2+ and K+ flux across cell membranes. Although a large amounts of phosphate presented in methylprednisolone preparations might be responsible for chelation of serum Ca2+.