Triac retriggering itself

[LectroNoob]

Hi,
I would like to know how can a triac retrigger itself.
I already know about dV/dT or phase shift, but is there another way?
It does that on a dimmer that I am working on and i don't understand why.
There is a pull-down resistor on the gate in case, but does not seem to affect the problem.

[capt bullshot]

The usual cause is dU/dt,
Try adding a snubber network (series R / C, ballpark values 10nF / 22R) across the triac, if this solves the issue, dU/dt was the cause.
 
In a dimmer circuit, look at what kind of load you've connected and wether the dimmer is suitable for the load. E.g. some (especially simple) dimmer circuits require the load to be mostly resistive (like a tungsten light bulb) and won't work well with a modern LED lamp.

[LectroNoob]

I already have a snubber across the triac. Something around 10nF and 100R. Tried to remove it to see if it got worse, but didn't change anything.
There is an inductor in line with the triac. I tried to short it with a wire but didn't seem to correct the problem.
I'm out of ideas

[capt bullshot]

Depending on your Triac and load, try a smaller R and larger C (e.g. 10R / 22n), sometime even larger C values will be required. Don't go for a too low R if you're using a small (less than a handful of Amps rated) triac.
Some interesting literature:

https://www.onsemi.com/pub/Collateral/AN-3008.pdf.pdf
https://www.st.com/resource/en/application_note/cd00004096-rc-snubber-circuit-design-for-triacs-stmicroelectronics.pdf

[LectroNoob]

I doubled the snubber capacitor and reduced the resistor to 10ohm, but no apparent change on the retrigger
Thank you for your inputs

[Neomys Sapiens]

Also Fairchild AN-3008 and the appnotes from Teccor.