That is a good question (( and hopefully at some point we will be able to figure out exactly what caused the spawning event. )).
Last fall my 75 (( different tank then mentioned above that had a spawning event )) which has two other Haddonis in it, had a temp spike up to 90*. Stupid me had pulled the temp probe out of the sump, and it was a cool night. I figured this out 6 hours later, so I am not sure how long the tank was at 90*. But, nothing happened, no spawning, and everything in the tank looked/acted the same as always. My temps normally go from 78*-83*, so maybe that played some role in nothing happening.