Plupload_token missing from request ... sometimes

I'm trying to upload a handful of images and all of the uploads work except one .. a different one each time.

After a bit more digging... it seems that (for some reason) some random upload POST requests are, well, different. I added a DPM to plupload_upload_access() and dumped the $_REQUEST var. On 3 of the 4 requests to /plupload-handle-uploads I see this:

[plupload_token] => HjmSMWV_yQxl_YvdqJfivwBF2QEkn7mRmqGVUq9TLWY
[name] => p18jbtl9p2vkkcemnr1656urt7.jpg
[SESSdb05ec8399870ea1b9b43ff038e87808] => piJmE43u4Tp72yDjRXZ9BbYZDRs2tc6ih-VrqeDMGE0
[Drupal_tableDrag_showWeight] => 0
[has_js] => 1

But on the one that craps out I see this:

[SESSdb05ec8399870ea1b9b43ff038e87808] => piJmE43u4Tp72yDjRXZ9BbYZDRs2tc6ih-VrqeDMGE0
[Drupal_tableDrag_showWeight] => 0
[has_js] => 1

Why would the token & name be missing?