Seeing the growing size of recent orders (and arguably pace of such announcements), it occurs to me that Tesla may be incentivizing early and large orders to effectively create a virtuous cycle of snowballing semi truck orders for Tesla. The more major players see other major players getting in line, the more real the potential of the Tesla semi seems, and the more real the risk of competitors gaining an advantage seems to those who haven't yet made a reservation. This would also be helpful to Tesla in allowing them to have initial production at a meaningful scale with a bulked up order backlog from incentivizing early and large orders (think initial production line scaled for 5K or 10K per year vs. say 500 or 1K).
If Tesla has been incentivizing early and large test fleet orders, here's my hypothetical of what may be going on...
Stage 1 ~now-2019,2020, various large companies will try out the semi with the 10-100 truck test fleets they've been ordering, and total initial orders and Tesla production on initial capex spending/production line capacity might total something like 5,000 units. In stage 2, ~2020-2021, Tesla would be planning to produce 20K-30K/year semis with additional capex commitment/production capacity, and will offer places in line and order sizes reflective of stage 1 ordering.
I'm not suggesting Tesla would cut anyone off from ordering, but, rather that there would be a line for stage 2 production reservations starting with these early stage 1 reservation holders, with Pepsi so far being offered the largest number to order (of the companies whose current order size is known). If and when the existing owner line was worked through, reservations would be lined up on a first come first serve basis. Stage 3 and beyond targeting expansion rate would be easier as the product and the market's interest would be more established.
fwiw, I just picked 5K and 25K from my gut... the point is really about building capacity in stages that match the trial process the customers want to use, without starting off with an inefficient low production volume.