I think they answer their own question in the text. However, the government and others probably want people hitting 65 and retirement to get sick, pay a lot of resources to Big Pharma, and the "sickcare" industry and die young.
As for previous vaccines getting activated. I do think this happens. If side effects are immediate people might (correctly) blame the vaccines. If an autoimmune condition or cancer is triggered later then the victim is unlikely to make the connection.
Here's a really evil , imo, vaccine recommended for those over 65. Guess what, it just happens to be "adjuvanted" with squalene?!
What's up with squalene? Well, according to the hypothesis presented in "Vaccine A: The Covert Government Experiment that's Killing Our Soldiers and Why GI's Are Only the First Victims" by Gary Matsumoto squalene was the adjuvant used in the new anthrax vaccine used on American and British soldiers in the Gulf War.
Mr. Matsumoto correctly predicted that this adjuvant would be rolled out on the general population. Here it is showing up in a flu vaccine "for those over 65".
Here's what the CDC has to say about an "adjuvanted" vaccine: https://www.cdc.gov/flu/prevent/adjuvant.htm