bobs,
My wife also went to fingerprinting twice but nobody ever told us why prior to the interview. Do not worry about the interview, at least they told you in advance what the reason was - which was not the case for us. Prepare the required paperwork as per the interview notice and go get your passport stamped! An added benefit of the interview is that you get a stamp in your passport and do not need to wait for your green card if you need to travel out of the country. Make sure you get police clearance notices for your spouse from all places of residence in the US (for her) over the past 5 years. This is done at the county level so you can go to any sheriff station or police department (but call in advance to know how long it takes to get it done, also smaller stations might not do this kind of admin work). You have to pay for the document (small amount). In our case we went there and were told it would take 2-3 hours to get the document which was a bit of a surprise. Our AO, who was the nicest person, told us it was not uncommon to get fingerprints rejected. Blame those small ravishing hands your wife has! Nice to chat with someone form Nepal by the way.
Cheers,