Honestly, depending on what they are being used for, transistors have a fairly high failure rate compared to other components in any electronic. One of the main functions of them is to amplify current, and any shorts in the system can cause a lot of stress on them. We haven't had a ton of SDS100s go through our repair department, and those are not commonly the reason they have.
It is possible it is a manufacturing defect; anyone who has worked in a factory can tell you mistakes can happen. it's the job of ISO compliance to minimize that, and it's a big part of why we have warranties. A transistor is likely to fail if any of the components in the circuit have a problem, so while they may pop up as a replaced part often, it can be caused by any number of failures. We track all of these failures and if there was any sort of common defect, we remedy them, but it does not seem to be the case here.
Most likely it's not anything you are doing with your unit, you can help ensure the health of your scanner by avoiding dropping it, making sure it has good air flow around it, and as hard as it may be, sometimes it's good to give them a bit of a rest.
