Examples of scarab not finding the path because of a moving target (red probe): [Replay]
What you can see on the most right image; moving objects (teal probe) blocking a scarab is way more abrupting to scarab pathfinding than static objects.
This is why most of the time when you aim a scarab at a center worker to get maximum damage; when your opponent moves all workers at once it will 'dud'. You dont necessarily have to go away from the scarab either. Some luck can play a role where it have the opportunity to go straight through all blocking probes or if the probes blocking stops, giving scarab pathfinding an easier time to navigate.
As seen on the images, static objects gets a try on pathfinding (red arrows) while moving objects blocking the path do not. Most likely because it never gets the time to pathfind around the blocking object as it keeps moving away from the scarab, resetting pathfinding algorithm.
So how do you actually get the most damage out of scarabs?
As seen in the image below, you can see the reaver can shoot through minerals if you drop it close enough to mineral line. This i would argue is the safest option to keep reaver both alive and do damage to whatever stack of workers you want to attack (as the scarab will jump through all minerals straight to target).
Next is the Green square which serves as the perfect position to get workers cornered. They have nowhere to go resulting in lost workers and hopefully many if aimed correctly (aim at a worker that isnt already blocked by another unit).
As the defender you have four options if a reaver comes:
1. Do nothing, let probes die until reaver is dead.
2. If reaver drops at the Green square you pull probes to gas and then to center map to keep them as unstacked as possible.
3. Flee to base as thats a long distance and you wont need to micromanage them anymore after that, probes will die as you will go straight through reaver (Green square).
4. Keep probes unstacked by going to Center map (yellow lines upwards)
I would say option 4 (yellow lines) is best because it will makes workers unstacked the quickest and starts moving asap. If reaver aims incorrectly (at a worker followed by another worker) it will most certainly 'dud'. Least workers will be killed.
[Replay]
And the scenario if reaver is too far from minerals: (green static target / blue moving target) [Replay]
I encourage you (who uses reaver a lot) to study your own games where you use reaver and see if you can apply this information somewhere to un-'dud' your games.
Maybe sometime in the future; all games becomes un-dud:ed. That would be really cool to see.
I guess thats all i had to say for now. Thank you for reading.