Understanding high-speed Improvements to discrete BJT multivibrator
I believe it's providing a path for the stored base charge to discharge. Basically turning the capacitors into &quot;speed up capacitors&quot; in addition to timing caps. It could be good to plot the current through the zeners to confirm.

a cap starts off discharged and starts charging up towards VCC when its transitor is an open switch

the base of its transistor eventually rises to turn on (thanks to r9), and closes its switch.

the diode side of the cap suddenly gets pulled low, making the other side a negative voltage.

the zener is now in reverse breakdown and pulls the attached base negative - sucking out all the base charge for a rapid turn off.

Repeat


The next question could be, &quot;does it have to be a zener?&quot; I keep oscillating back and forth as to whether a regular diode would do the same thing. Since you don't use the reverse breakdown part of the zener diode, maybe it doesn't need to be a zener diode. Plotting current through it will help illustrate that.
EDIT ha! I originally wrote that it had to be a zener, and as I was editing my answer to say it doesn't have to be, you went and showed that it does get faster with lower voltage zeners. Ok, so there's still something to think about!