How much does the temperature change in your garage (or wherever you are storing the acid)?
If you close the barrel when it's warm and then it cools down, the pressure inside will drop significantly, causing it to crush in. Or, if the seal is not great, then when the barrel warms up some vapors can escape, and then as it cools the lid is pulled back down firmly into place as the pressure inside drops (kinda a one-way valve).
Hydrochloric acid will do this more than water (and less than gasoline) because of the relationship between vapor pressure of the liquids and temperature: if the temperature varies between 20 °C and 30 °C, water's vapor pressure only changes by about 10–10 atmospheres (0.000000001% reduction in pressure inside). In contrast, concentrated HCl (35%) will change by 0.08 atmospheres (8.0% reduction in pressure inside).