What are common causes of unexpected ligation products?
Could be insert polymerization.  If you have your stretch of DNA like this:

 (5')-AATTagctagcatcgtgatcgacg-(3')
          ||||||||||||||||||||
     (3')-tcgatcgtagcactagcagcGGCC-(5')


And you take that, flip it around, it will ligate onto itself, like this:

 (5')-AATTagctagcatcgtgatcgacg-(3')(5')-CCGGcgacgatcacgatgctagct-(3')
          ||||||||||||||||||||              ||||||||||||||||||||
     (3')-tcgatcgtagcactagcagcGGCC-(5')(3')-gctgctagtgctacgatcgaTTAA-(5')


This is easy enough to check for.  Just calculate the size fragments you'd expect from a vector with two inserts (there'd be an extra BcII cutsite now, and the extra fragment would probably be the size of the insert).  If this is just out of curiosity, and one of your things is the correct size, I'd ignore the ones that didn't work and go ahead with your ones that did.

If you do have a problem with it, or you are getting all your clones polymerizing, you can treat your insert with an alkaline phosphatase.  This will eat off the (5') phosphate that the ligase needs to stick the on to the (3') hydroxyl.  The result is that an insert cant bind to another insert, but once it finds a vector it will be able to ligate:

 insert to insert:    AATTagctagcatcgtgatcgacg-(3') :( CCGGcgacgatcacgatgctagct-(3')
                         ||||||||||||||||||||              |||||||||||||||||||| 
                    (3')-tcgatcgtagcactagcagcGGCC  :( (3')-gctgctagtgctacgatcgaTTAA

 insert to vector:    ....ctagct-(3')     AATTagctagcatcgtgatcgacg-(3')(5')-CCGGcgacg....
                      ||||||||||              ||||||||||||||||||||              |||||||||
                      ....gatcgaTTAA-(5')(3')-tcgatcgtagcactagcagcGGCC     (3')-gctgc....


There will be nicks on the top left, and bottom right, but the bug you put it into should clean those up for you.  You can get alkaline phosphatase for fairly cheap from lots of places, NEB is always a safe bet.