I've found that Wishing Star is much much much more likely when you're on extremely low health. When Omega Weapon Mediggo Flamed me and I ended up on 1 HP, I was like "HAH! RINOA INVINCIBLE MOON TIME! (Rinoa -> Limit Break -> Angelo)" And then she Wishing Stars and I'm like

I tried again twice to get the same result. On the third attempt at beating Omega Weapon, I got Terra Break-ed and Rinoa ended up on 200-300ish HP and I got Invincible Moon.

I reckon that..yes, it is random to a certain extent. But lower HP and whether or not you have Aura will affect it as well. I think I can back this up with Squall too, as Lion Heart also seemed to come out more often after Ultimecia's final form Hell Judgemented me.