Why is Initiator column sometimes blank

When looking at the Initiator column in the CSV file e.g.
https://www.webpagetest.org/result/160906_5V_J7B/.com_requests.csv

Sometimes it is blank. How can this be?

Chrome doesn’t provide the information in all situations

If you load the page in devtools and look at the initiator column it will sometimes say other for example look at the request for http://t1.stormiq.com/px.gif