I noticed that in my BWA alignments, given a read that is mostly well aligned but with a few bases at the end mismatched, sometimes it's labeled as a mismatch (M), while at other time it's a softclipping (S). What's the deciding factor for mismatch vs soft clipping?

For example, I have seen cases where two 76bp reads are fully aligned except for the first base, yet in one case the first base is a called as a mismatch, but in the second case it's soft clipped. I wonder what are the possible reasons for such difference.