If the second doses are due now, why are they not being given now? This "holding back" still doesn't make sense.
The only thing I'd add to what @ian94 has put, is that there's probably also a degree of insurance to it as well. Saving ~100,000 jabs each day builds up a stockpile giving us a few days extra supply of second doses (more so in the short term reflecting the situation 12 weeks ago where it was still fairly low numbers vaccinated daily) in case the Pfizer supply doesn't come back online as quickly as had been hoped. If it does come back online as quickly as is hoped (which for our, the EU's, and Canada's sake let's hope it does) then great, we've got a moderate stockpile that we can slowly put back out to vaccination sites to use up. If it runs into teething issues (see: Astrazeneca vs the EU), we've got enough stockpiled to hopefully ride out the period of lower supply (in combination with the ability to push bookings from the generally 10 weeks arranged to 12 week limit) until they recover