<p>A few opinions on the sentences, with guidelines that are probably safe:</p>
<ol>
<li> “is because” is usually wrong. “Is” is a linking verb. The construction “A is B” should mean either that A and B are the same part of speech, being equated with each other, or that B modifies A (generally true, possibly some exceptions). In this sentence, the subject (A) is “shift.” The clause starting out “because” states a reason. The shift is not a reason, the shift is happening for a reason.<br></li>
</ol>
<p>This explanation might make you think that "The reason is because . . . " is an acceptable construction, but the SAT writers definitely do not like it either. The correct form is “The reason is that . . .” I suspect that “reason” needs to be equated to a clause that functions as a noun, and “that” clauses do so, while “because” clauses essentially function as as adverbs. But more discussion on this would be welcome.</p>