For each effect you'd like to control you'll use 3 modulators, a random value, a macro knob, and a button.
In Glitch2 set the dry/wet value of an effect you'd like to control to 0%, or all dry.
Map the button macro so when on the mix is 100% (or whatever you'd like) and 0% when off.
Map the random modulator to the button at 100% modulation strength.
Map the macro knob to the button at 100% modulation strength.
Ensure the random modulator is set to unipolar and choose a time interval to evaluate a new random value.
Since unipolar random values are 0 to 1 and we are mapping this to a button's on/off state and the button turns on at .5 or above, assuming even distribution of the random values, by default this will give us about a 50% chance the button is on for each time cycle of the random value generator.
We can adjust the likelihood of the button being on by using a macro knob in bi-polar mode and adjusting the knob to control the chance. If we set the macro knob to -50% then the button should never be on, and moving it to +50% will have it always on. This is the offset to allow you to dial the % chance you'd like the button to be on.
Add a set of modulators like this for each of the effects you'd like to have a chance to trigger and then ensure they have an active state on the Glitch2 sequencer.
Let me know if you need clarification.

