In the tricium-dev logs (https://console.cloud.google.com/logs/viewer?project=tricium-dev):

2018-08-10 15:50:49.543 PDT
[driver] Trigger request received. :: {"run ID":5632747150245888, "worker":"RecipeNoOp_UBUNTU"}
2018-08-10 15:50:49.550 PDT
[driver] PubSub userdata for trigger: "CICAgKjC3oAKEig3ZjE4NmIzMzAzOGVmMDAyNWY5OGMzMjUwMWI0NTU1Zjc2MjFhNjExGhFSZWNpcGVOb09wX1VCVU5UVQ=="
2018-08-10 15:50:49.597 PDT
[driver] Failed to call Driver.Trigger. :: {"error":"failed to trigger worker: failed to call trigger on buildbucket API: failed to trigger buildbucket build: googleapi: got HTTP response code 404 with body: 404 Not Found\n\nThe resource could not be found.\n\n   "}

It's possible that something with the request might be wrong; it might be the case that more clues could be found in the buildbucket (cr-buildbucket) logs.