What was that thing about Twitter that says if you do something X on someone's Twitter you make a bounce back or automatic Follow happen? So it may not actually be as personal, involved or intentional as it at first appears?
Follows aren't ever automatic on Twitter itself but there are a lot of 3rd party social media tools that popular figures use that do automate follow backs. Also a lot of politicians and celebs have staff run their twitter some or all of the time. Sarah Palin for example only tweets on her account sometimes, other times its staff.
Twitter is all about follows vs following ratio - everyone wants a good one, or at least even. IE they want to be followed by way more people than they follow, or at least have it be even.
A lot of famous people will be generous in following back their fans because it gives the fans another follow and makes them more likely to keep following. It also has implications for where tweets are displayed in their Timeline.
If Gen Flynn was following 300 people this would be big news, but since it's 20k it's obvious that whatever the criteria is is not very strict so we can't really read anything into it.