This awkward sentence uses factor in what is sometimes called the ’middle voice’, because it is halfway between the active and the passive.
This construction takes a transitive VERB and ’recategorizes‘ it as intransitive, with what is ordinarily the Direct Object (or Patient) acting as the Subject (or Agent):
ACTIVE: subjectX VERBS objectY. ... Danny is cooking the eggs.
PASSIVE: objectY is VERBed [by subjectX]. ... The eggs are being cooked.
MIDDLE: objectY VERBS. ... The eggs are cooking.
This is not exactly rare in English, but it is not common, either. Only a few verbs and verb expressions are used this way. They are called ergative verbs; there is a brief description of their use in English on Wikipedia.
In this sentence the expression factor into is employed ergatively. The ’canonical‘ sentence which underlies the sentence is not in the active voice,
[subjectShawn White] [trans.verbmust have been factoring] [dir.obj.how much he has to risk] into his decision.
but the middle voice:
[subjectHow much he has to risk] [intr.verbmust have been factoring] into Shawn White's decision.
This sentence is then inverted to put the ‘new information’ into the strong final position:
But factoring into Shawn White's decision must have been how much he has to risk.
You will encounter constructions like this from time to time, so you need to know how to read them. But I do not recommend that you put any effort into mastering them in your own speech and writing. In this particular sentence the combination of middle voice and modal past progressive is way too complicated and hard to follow, and it reads† very affected. Moreover, has to risk is ambiguous—has to is easy to read as must.
A simpler way of expressing this would be to emply factor as a noun instead of a verb:
One factor in Shawn White's decision must have been how much he has at risk.
† This is another instance of middle voice. You often run into “This book reads well” or “This car handles easily”. There’s yet another problem with when you should use an adverb and when an adjective complement—one more reason to avoid this construction.