Suddenly SQL server restarted with pending task count increased
I would start by saying that best person to analyze dump in MS guy or person who knows about it. I would just try to point out some basics from the log you posted

At 19:46 maxworkers- 2944 workers created 789 oldest pending taskwait time 0 pending task 4
At 19:51 maxworkers- 2944 workers created 982 oldest pending taskwait time 256987 pending task 165

Please see that taskwait time 256987 and pending task 165. This means the scheduler was hung and 165 tasks were waiting on it to get a scheduler and run. In this case MS SQL Server was not able to get out of this hung scheduler scenario and waited for a while but again it decided that it would be best to restart itself to come out of this hung scheduler scenario and hence it restarted. Now for reason why it hang is beyond my ability to tell with the information you have posted.
Also note a thread is assigned a scheduler and it should run on that scheduler and that is why so many threads were waiting on this hung scheduler.