It makes perfect sense, because Jason Castro - for 2013, at least - established himself as one of the premier offensive catchers in the game. We talked about this in his Exit Music (For A Player) post last month - Castro ranked 4th among all catchers in baseball (3rd, if you take into account that Joe Mauer will be the Twins' 1B1 in 2014) in WAR, and 2nd among catchers in Slugging, just one point behind Colorado's catcher.
So that teams are interested in Jason Castro is no surprise. The Astros have a recent history of trading anyone who has value, and given that Castro is young - he'll be 27 next June - and will be arbitration-eligible for the first time (and won't be a free agent until 2017). So to recap: Castro is a young catcher who broke out in 2013 and will be cost-effective at least for the next three years, presumably.
But he did have that knee injury which cost him his 2011 season, spent some time on the DL in 2012 due to the very same knee, and there was the cyst on the same knee (which has already been established as a Not That Big Of A Deal injury) that shut him down at the end of 2013.
The Astros do also have Max Stassi waiting in the wings, as well as a valuable backup in Carlos Corporan. This doesn't take into account Tyler Heineman, a 22-year old catcher who has a career .311/.393/.460 line between Tri-City and Lancaster in the last two seasons, but is still likely at least one more season away from contributing at a Major-League level.
Keeping in mind that Sheriff Brian McCann and Jarrod "Leg Hair" Saltalamacchia are on the market, and teams that want a premier offensive catcher who have prospects to deal (and maybe don't want to give McCann $75m) are obviously going to check on Castro's availability. You can bet that any deal involving Castro will be a big one (return-wise, anyway) - not at all similar to the Bud Norris trade.
Perhaps Castro will have to move to a different position to protect his knee. Perhaps 2013 isn't the Real Jason Castro. Perhaps the Astros like a 22-year old Stassi just as much as they like Castro - after all, Luhnow acquired Stassi, he inherited Jason Castro. Perhaps the Astros have also lost 100 games in each of the last three seasons, each one more crushingly depressing than the last, and would like to hang on to Castro. Perhaps Castro is one of the best offensive catchers in the game. I'm not advocating one way or the other (yet), but trading Castro is a big gamble.