As has been stated before, it doesn't take a tragic fall. Just a really bad stroke of dumb luck to enter the shadows.

How about dumb luck and a tragic fall?

No you most definitely aren't, i also build most of my characters like that(all that aren't dicepool exercises of somekind)
Well to be precise, i make up character of some kind(from a pretty long piece of background to few word concept) and then build her witch usually ends up influencing the background, so i then tweak to background witch ends up influencing the build and so on, it's vicious cycle i tell you

How about some tragic luck?

Yes. It always happens. There is no end to it!

The most annoying part for my main character build is that they changed the rules for karmagen between when i started building her and now
so know i have to modify a 1000 karma build of her with the new costs

Kinda funny to see that the bulk majority of mages doing shadow work are doing shadow work because they can't do corporate work.

I really expected to see a few backstories with their mages having moral issues with working for a corp. Heck, a ton of the traditions and mentor spirits have honor codes that don't allow for a lot of the actions and work that a corporate mage might be required to do. Wouldn't they have some sort of inner conflict if their orders/research contradicted their tradition's values? Wouldn't they lose magical power if they acted in a way that went against the teachings of their tradition/totem/mentor spirit?

The most common magical tradition in the corp world is the hermetic, which sees magic as a science for the most part. While some mages certainly would have moral qualms, it is feasible that most would see their darker pursuits as justified.