They used to give AZ to mostly younger people as the side effects where seen to be stronger than for the PFE.
A fear could be that when they start to give the PFE to younger people, they will see the same stuff as with AZ as it triggers the same immune response.
Pfizer has been distributed to many young people in other countries though and no such issue has popped up. Not saying it's outside the realm of possibility, but I think it's increasingly unlikely. Plus it'd be no surprise to me for the mRNA vaccines to have somewhat different side effect profile than the "traditional" ones.
A fear could be that when they start to give the PFE to younger people, they will see the same stuff as with AZ as it triggers the same immune response.