As the other answer misses a bit of the information I was looking for, I thought I'd add the results of my research in case anyone else is curious. Looking in

C:\HOME_DIR.m2\repository\org\springframework\boot\spring-boot-maven-plugin\VERSION_NUMBER\META-INF\maven\plugin.xml

gives the configuration file for most of the related goals for spring with maven. Specifically, the snippet below describes how the spring-boot:run goal works.

<mojo>
      <goal>run</goal>
      <description>Run an executable archive application.</description>
      <requiresDependencyResolution>test</requiresDependencyResolution>
      <requiresDirectInvocation>false</requiresDirectInvocation>
      <requiresProject>true</requiresProject>
      <requiresReports>false</requiresReports>
      <aggregator>false</aggregator>
      <requiresOnline>false</requiresOnline>
      <inheritedByDefault>true</inheritedByDefault>
      <phase>validate</phase>
      <executePhase>test-compile</executePhase>
      <implementation>org.springframework.boot.maven.RunMojo</implementation>
      <language>java</language>
      <instantiationStrategy>per-lookup</instantiationStrategy>
      <executionStrategy>once-per-session</executionStrategy>
      <since>1.0.0</since>
      <threadSafe>false</threadSafe>
      ...
</mojo>

Specifically, the <executePhase> tag (detailed partially at https://maven.apache.org/developers/mojo-api-specification.html), which (I believe) lets this goal execute a different phase as it's run.

I missed this detail before (it doesn't seem to be documented anywhere very well either). Either way, this explanation is satisfactory enough for me. If anyone finds better documentation for this tag, I'd appreciate a link :)