Powder vs tablet chlorine for pools

Factual Questions
1

I worked for a pool company briefly about 11 years ago. Put the two together and they ignite. I remember an irate customer calling threatening a lawsuit. I gave the call to the manager, who read him the warnings clearly written on the containers. I have been trying to tell this to my phd. chemist friend and get answers, but I don’t have the right details to tell him. One is blah blah calcium di-chloride, the other is something calcium tri-chloride.

Why and how does it ignite? Explain it to me so I understand, beyond just putting them together. Then explain it in the terms of a chemical reaction, so I can relate it to him.

2

Hmm…ignite? As in start a fire? I have never heard this, and I worked at pools for a number of years (prestigious, no?). Most powders and tablets are anhydrous sodium hypochlorite IIRC, but there is a huge variety, so you’d have to narrow it down. You’d have to give the exact type of chlorine to get an exact answer.

The main danger I’ve been warned of associated with pool chlorine (in whatever form, liquid, granulated or tablet) is that it can form chlorine gas (Cl2), which is toxic. This can happen (I believe) by mixing it with ammonia.