I suspect anyway, that it is a natural result of this site being opened.
- Web Apps was the first site opened via Area 51
- Before that, there had been a number of (off-topic) web app related questions on Super User1
- Web Apps has been a migration target from Super User from the beginning
What's probably happened is that there was a lot of pent-up demand and, once this site opened, Super User finally had a place to send decent but off-topic questions.
After seeing a couple of your examples, it's clear. Those questions pre-date the creation of Web Apps. So...
- Web Apps opens
- People go to Super User to find questions for Web Apps and ask for them to be migrated. (There didn't used to be age limit on migrations.)
Since the meta data of a question, like the date/time it was created, don't change in a migration, if you sort strictly by age many of those questions will be from before Web Apps existed to accept questions of its own. (The private beta of Web Apps began June 30, 2010. By definition any question with a date before that will have been migrated from elsewhere.) The same will be true of many of the early Area 51 sites. (Lately, however, the policy has been not to migrate questions to sites that are in beta, so you won't see it as much or at all at other Stack Exchange sites.)
1 A considerable motivation for creating Web Apps.