It's not that hard to predict... If a group of pvpers without weapons breaks line of sight or comes from off screen then rushes you and whips out wands, there is only one logical explanation.
And if you mean that its impossible to know who the target is, all of the mages still have to look directly at whoever they are syncing the explo on, and as long as your group isn't very tightly bunched together this shouldn't be too hard to see. You can also see the first explo or two bounce off the target if they have reflection up, which helps in countering gank squads that roll around with explo loaded simply to use this tactic as a preemptive strike.
I honestly don't think there was a problem with wands before this patch. They allowed for more options for defensive builds in terms of clutch kills, enough to make playing that sort of spec that much more interesting when almost all pvp builds are 100% offensive.
Before the patch a fireball wand had a cast time relative to the spell itself, but shorter, same with Gheal wands. Now both wands are almost exactly the same "cast movement delay" time. This simply doesn't make sense.
If the change in movement delay was implemented for nerfing lightning/fireball wand syncs as a couple people have suggested, it seems to me that the effect will be just the opposite, instead nerfing solo fielders.
If a group of 3 stun mages (not necessarily alchy mages) were to attempt to pot/wand a fleeing soloist before this patch, the soloist should be able to keep only one or two of them on screen at a time, thus being able to chug/bandage/minihealwand or chug/ghealwand through it for a good while if they know what they are doing and are not inebriated.
Now after the patch, with 1 or 2 stun mages chain potting the soloist, they will eventually gain enough ground on the soloist to utilize lightning wands because the soloist will eventually have to cast/wand gheal or mini heal, even if they are a healer with gheal pots. This will result in the third pvper in the gank squad catching up no matter what because he is not delayed in any way, and the first or second attempting to stun then transitioning to explo+light wands again or a different sync if that is their forte, or back to basic potting because stun failed and the soloist is still running.
I'm not sure if anyone understood or read that wall of text, but it seems to me this will have the opposite effect intended. My suggestion would be to revert back to the regular wanding system but have a mana cost on them of 1/2 the spells cost, either rounded down or (rounded down)-1. Either that or just change it back the way it was. In my humble opinion, if a soloist is confident enough to carry around 20+ gheal wands, he should be permitted to escape a gank for as far as his skill and his mount's fatigue take him.