Is there a common reason for a Macbook battery to disconnect from the logic board? I changed my Macbook Air's battery one month ago. It is a mid 2011, 11".
I've changed it for a cheap battery bought on Amazon :  

It worked fine for one week, then the battery started disconnecting on a regular basis from motherboard. The little connector from the battery to the board "jumps" pretty often from its place. I then can use the Mac only if plugged in (the battery icon has a little cross on it + message "no battery available"). Until I reopen it, and reconnect the battery. Pretty annoying.

I wonder if it is more likely that :


Shocks do that. Not very likely to me as I tried pulling on the connector with my finger to see if this can happen.
Some "ejection system" kicks my battery out. As if the system detected overheating or such thing and decided to eject the battery.
Any other reason ?
It's option 3 - "Other."  There are two possibilities for your battery becoming disconnected with your logic board and both have to do with the molex connector:


The connector on your battery is broken/defective.  If this is the case, the warranty on the battery (provided you are still within the warranty) should get you a replacement and have this fixed.
The connector on your logic board is broken/defective.  If this is the case, the only surefire way to fix it is to have the connector replaced.  This will involve soldering of the logic board.





If repairing/replacing the logic board is out of the question and the everything works fine with the exception that the connector won't stay connected, you can secure it in place with some Kapton tape.  It won't be the most elegant solution, but for under $20 for the roll of tape, it's a much more cost effective solution.



As for your first two speculations...


Shocks. The amount of destructive force your Macbook would have to go through to "shock" the connector out would destroy your Macbook completely.  
Ejection system.  You would see some sort of mechanism to do this.  If your MBA senses things are getting too hot, it shuts the computer down.  There's nothing in any computer that "ejects" connectors.