What could be causing my bath salts containing baking soda to explode out of their containers?
I suspect the seasonal introduction of humidity is the root of the problem, which is readily taken up by the magnesium chloride. Then, the following possible reaction:
$\ce{ MgCl2 (aq)  + 2 NaHCO3 (s) -&gt; 2 NaCl (aq)  + Mg(HCO3)2 (aq)}$
Now, aqueous Magnesium bicarbonate exists only in solution (so decomposes on drying), is not stable on standing (per my experience), or warming (whereupon it also decomposes):
$\ce{ Mg(HCO3)2 (aq) -&gt;  MgO (s) + H2O (l) + 2 CO2 (g)}$
where the pressurized buildup of carbon dioxide gas is likely causing a problem.
Solution: Try adding a drying agent or removing the very hygroscopic Magnesium chloride from your formulation.