The bar for recall is actually relatively low. Basically, when you find a fault (somehow, maybe in regular QC in manufacturing, or something bad actually happens in the field, or some engineer is fucking around), the question is "can this affect patient safety/outcome in the field". If the answer is at all not a certain "no", then you're probably thinking recall at that point, unless you can adequately root cause and contain it.
Since the nature of "oh oops" is that they tend to affect systems in ways that are not anticipated, there's often insufficient evidence to rule out danger to patients, and therefore there's a recall.
For example, if you sold 10,000 diagnostics machines, and then discovered that because of stack up of tolerances in electrical components, something like 1 in 100,000 machines will have a fault that affects the customer safety. However, because your original analysis (during design phase) did not show this problem, you never bothered recording the actual performance characteristics of 40% of the components involved in the stack up.
Now you're in a pretty awkward situation, that could result in a recall. And it could very well by that all 100,000 machines sold are just fine.
