What happened to Mass Effect 2?

[Reasonable]

Not sure there's a problem. Here's the data on the titles I can see:

http://www.vgchartz.com/swlaunch.php?reg1=All&game1=Mass+Effect+-+X360[7635]&reg2=All&game2=Mass+Effect+2++-+X360[28815]&reg3=All&game3=&weeks=100

now this week by week view shows the weekly average has dropped faster (so far)

http://www.vgchartz.com/swlaunch.php?reg1=All&game1=Mass+Effect+-+X360[7635]&reg2=All&game2=Mass+Effect+2++-+X360[28815]&reg3=All&game3=&weeks=100&weekly=1

however, unless it really collapses at retail it should sell at least the same if not more than ME. I doubt it's going to sell a huge amount more, but then I don't think that's a reasonable hope. ME1 sold very well to the RPG market on 360 and I don't think that market has actually gotten that much larger so ME2 simply sold more up front as a trusted IP vs a new one and is showing slightly quicker drop off as it doesn't need word of mouth, etc. unlike ME to give it a stronger tail.