A possible reason for the auction end time is that if you use Turbo Lister (eBay's free tool) your options are:
1) Auction starts when you upload.
2) Delayed auction start time. This start time is only available in 15 minute increments. 12:00, 12:15, 12:30, etc.
That could explain why the heavy load on the hour and half hour.
Just my two cents worth.