Who needs reasons? CR woke up on the seventh day (year, actually) and decided to give the Constellation a S3 shield, and all was good...
Seriously, the mid-size ships with S2 ships have such a wide range...I can't get the Starship42.com Fleetview to work for me right now (for some reason), but the ships with S2 shields range quite a gamut...you have:
the Cutlass with 1 S2,
the Freelancer(s) with 2 S2's,
the Vanguard(s) with 2 S2's,
the Mercury Star Runner with 2 S2's
the Retaliator with 2 S2's...
...
and now the Connie with 1 S3. This gives them a bit more of a range/steps along the ladder to positions ships. And the Connie is a pretty big ship. Based on this, it would be nice if the other bigger ships with S2's got another look, like the MSR and Retaliator...they are similar in size to the Connie, and the Retaliator is supposed to carry 6/7 people, why not give that a S3 shield? Oh yeah, anybody with any brains melted their Retaliator long ago; nobody owns one any more /s