That's true, but there seem to be two problems with explaining the data that way.
Firstly, the second shot graph shows the same thing: a clear wave that peaks 10 days after vaccination.
Secondly, if that were the cause you'd expect to see a very different almost square wave like graph, with a steady background rate of cases that the shot failed to stop followed by a clear drop as the cohort moves on more or less simultaneously. The data doesn't show that.
It seems in Alberta the window between first and second shots is 8 weeks or 56 days. I've been using two weeks as an example but that's too short.
https://www.albertahealthservices.ca/topics/Page17389.aspx
If we select the 56 day period (the graph is zoomable by dragging a region), then we see a large wave that starts near zero, peaks near day 10 and then by day 56 has dissipated. This cannot be the result of people moving into the other cohort because it's all over before anyone becomes eligible for the second shot.
W.R.T. reactions - it's interesting how much people's experiences here vary. Almost everyone me and my girlfriend know locally who took the vaccines have had some kind of bad side effect. I suspect there may be several causes of this divergence:
1. Differences in brands used. Moderna is 3x the strength of Pfizer. All data shows Moderna causes way worse side effects and especially after boosters. But, some places use Moderna a lot (like where we live) and others don't. Of the people I know who live in the UK, where AZ was standard, none have reported any side effects.
2. Differences in age groups? Side effects seem to be worse or at least more noticeable amongst the young.
3. Filter bubbles. Our friends know we didn't take the vaccines (so far), so we're "safe" to talk to about side effects and reactions. If you're the sort of person who has publicly proclaimed that you Fucking Love Science!™ - as many people have done - then it seems likely people won't want to mention any effects they did have with you.
It's especially easy for that to occur because all the bad reactions we have heard about are the sort of thing that isn't going to be immediately visible if you meet someone: a day off work due to "sickness", disrupted or stopped periods, and in one case sadly a heart attack. That sort of thing is invisible if the affected person doesn't confide.