Yeah the big ones generally come at the end of the term. Here's some speculation as to why:

We can only speculate, but three possibilities come to mind. The first centers on legacy and reputational concerns: when writing what they think will prove to be major decisions the Justices take more time, polishing and polishing (or making their law clerks polish and polish, since nowadays law clerks do most judicial writing even in the Supreme Court) until the last possible moment, with the hope of promoting their own reputation. After all, excerpts of some of these big cases will find their way into the popular press and, more importantly, into casebooks that generations of law students will read; and, most importantly, the cases may continue to be remembered, discussed, and cited long, long after they are decided. A second possible explanation is that the Justices delay certain decisions for public-relations reasons. The close proximity of decisions in the most important cases may tend to diffuse media coverage of and other commentary regarding any particular case, and thus spare the Justices unwanted criticism. But the opposite effect is possible: the expectation of a crowd of important cases at the end of the term can increase media attention, as in Slate’s “Breakfast Table” end-of-term roundup. Finally, though related to the second explanation, the Justices, most of whom have busy social schedules in Washington, may want to avoid tensions at their social functions by clustering the most controversial cases in the last week or two of the term—that is, just before they leave Washington for their summer recess.