The SCt. ruled on cases brought before by states aggravated by the voting "restrictions" placed upon them following the civil rights movement, where it was clearly shown that several states engaged in active suppression of black voters. So yes, you're absolutely right.... they can't just up and say one day "Hey, let's issue a ruling on ____."
That being said.... why did those states under restriction bring their cases before the SCt. following those historic voter turn-outs for Obama?
"Gone is the provision of the 1965 Voting Rights Act that prevented certain states, primarily in the South, from passing voting laws without approval from the U.S. Department of Justice. The U.S. Supreme Court ruled in 2013 that the tactics used to disenfranchise voters were a relic of the past and pre-clearance no longer was necessary."
https://www.nbcnews.com/news/nbcblk...uld-affect-millions-african-americans-n639511
Note the date of that decision.