9-0 SCOTUS decisions are not "extremely rare." In fact, most years 9-0 decisions represent the majority (or largest plurality) of cases. Most SCOTUS cases are about mundane elements of the law, about which the justices are often in agreement.
5-4 decisions are almost always the rarest decisions, but they often get the most media coverage because of the subject matter of those decisions.
Regardless of the facts involved in the case, the legal argument in the case sparking this post was rather esoteric and boring to non-attorneys. It dealt with the legal doctrine of "exhaustion of administrative remedies. Plaintiffs generally must exhaust all administrative (i.e., executive branch) avenues for resolution of their dispute before being able to avail themselves of the judicial branch. It's a separation of powers concept, but also, and more importantly, a judicial economy issue (i.e., shoving as many litigants as possible off to administrative law judges makes more room on judicial branch judges' dockets for other litigants).
Here, the plaintiff brought suit under both the IDEA and ADA acts with a list of requested remedies to redress his harm; amongst those requested remedies was compensation and lost income. If all of the requested remedies were recoverable under the IDEA act, then the plaintiff would have had to exhaust all administrative methods of resolving his claim before bringing suit in federal court; however, since compensation and lost income are not remedies available under the IDEA act (but possibly recoverable under the ADA), the plaintiff was not required to exhaust all administrative avenues before filing suit.
Again, most SCOTUS decisions are as boring as you probably found reading the above synopsis.
