The NTSB has just issued the Final Report with Probable Cause on this accident:

Probable Cause and Findings
The National Transportation Safety Board determines the probable cause(s) of this accident to be:
The tow plane pilot's loss of airplane control for reasons that could not be determined based on
the available evidence. Contributing to the accident was the glider instructor's diversion of
attention away from the tow airplane which resulted in his delay in releasing the tow rope once
the tow plane was no longer visible from the glider cockpit.

Full Report:
https://app.ntsb.gov/pdfgenerator/Re...Final&IType=FA

On Sunday, March 10, 2019 at 7:01:48 PM UTC-5, Duster wrote:
Though the final report w/any probable cause is scheduled for this Wednesday (3/13), there are a few details that perplex me. Video analysis (segment #2)indicates the spoiler handle starts to move aft at (in seconds?) 5.14s, then is near full aft by 9.21s until 11.13s when it "starts to move forward". Near the end of the epoch, the release handle moves several inches, then dangles, coincident with a snapping sound. The glider PIC reported deploying brakes when he saw the towplane below to the right with slack in the line. He stated,"and the[n] released the towrope." Partial to full brake handle aft for about 6 seconds, most while tug/glider still connected it seems (tho maybe wrong). Witnesses told the PIC they didn't think he pulled the tug's tail up; some of the written docs bear this out. Video stills recorded the tug's elevator position.