Why would bank foreclosure 2nd, but not 1st?

[T Handy]

I've bought at Sheriff's sales before, but only firsts. Several times in just the past few weeks, I've seen a major bank holding both the first and second mortgage foreclose on the small second but not the big first. Maybe the left hand doesn't know what right hand's doing, or maybe it's because payments are more current on the first? I know the Sheriff's Sale of the second won't wipe out the first so they're covered either way, but why would they do this? Could they have some strategy for this or is that giving the banks too much credit?

Though I doubt they're this clever, I wonder if the bank might be trying to find a buyer with funds to buy the smaller second, thus a good candidate to slide into the first? Or maybe they figure they can dupe someone who thinks it's a first into bidding and buying it at a low price, then they POW!, foreclose the first, pocket the cash, move on? I think if I did buy the second, I'd get paid off when the first foreclosed, but I'm not even sure of that.

I'd call the bank to find out, but they won't talk to me about a loan unless I have the owner's authorization to do so, which I don't have for these situations. Have you bought a second behind a first at the sale and if so, what happened with regard to the first?