Created a multiconfig job that builds on child nodes.
If I manually kick off a build, the "BUILD CAUSE" environment variable on the matrix shows MANUALTRIGGER yet each of the downstream builds show the cause as UPSTREAMTRIGGER.

Since individual builds cannot be kicked off manually on a node, it will always say UPSTREAMTRIGGER.
The BUILD CAUSE on axis builds should always inherit the value of the BUILD CAUSE of the matrix build.

This is similar to JENKINS-14438 which involves Run Conditions plugin but should really be extended to the Jenkins core.