Why did my developing tank almost burst open when using citric acid as a stop bath after developing with caffenol delta recipe?

When an acid solution such as citric acid and an alkaline solution such as Caffenol developer are combined, chemical neutralization results. A salt solution is the product and carbon dioxide gas is liberated (abundantly). Similar reactions occur in baking. Such reactions cause bread and biscuit to rise (leavening). Let me add, generation of gasses during film developing should be controlled. That's why we normally use a dilute solution of acetic acid (vinegar) as the stop bath. What can happen is, blisters often form within the film emulsion if the stop is too strong. This action will spoil the film. Let me add, development takes place in an alkaline environment (some exceptions). You can use plain water rinse to purge the developer or a mild acid rinse. Acetic acid is preferred if developing is to be halted quickly. Acetic acid is one of the ingredients of the fixer; thus we use it to not only stop developing but to reduce fixer contamination with carryover developer.

Going from developer to stopper means going from basic to acidic milieu. Washing soda is basic because it is a salt of sodium from a comparatively weak acid. If you add a stronger acid, like citric acid, it will cause the carbonate ions (CO3--) to get protonised and then essentially fall apart into CO2 and H2O, with the carbon dioxide bubbling out.

In contrast, caustic soda used for generating basic milieu has OH- ions to start with, and protonising those just results in H2O which remains in solution.

Personally, my private guess would be that a basic agent that tends to bubble when neutralised is a bad idea for working with film emulsions but that's nothing more than a gut feeling without any supporting experiments of my own.

When combining the agents you mention, this reaction is expected. But I would think that the person designing this workflow did not intend for large amounts of agents to be combined in this manner.