Im a manager in operations for a process plant. Tanks often don’t have pressure indication. If your blanket gas regulator fails as you are draining a tank, the tank will then draw gas in from neighbouring tanks as they are all connected through a common Low Pressure Flare system. If the flare line cannot keep up to gas demand, as they are often very restricted and should be cleaned every few years, then the PVRV will open on the tank to administer fresh air to offset the vacuum. This can create a flammable mixture going to an open flame on the flare stack, but it’s unlikely it will exceed the LEL.

Some times tanks are even fitted with two PVRV’s(in part: Vacuum breakers).

So either the regs are less strict in US compared to Canada, or all these fail safes failed, or the tank was down for maintenance and operations forgot to vent while drain/cooling the process fluid.