So why were the adjustments made?
The link I provided is a presentation by the NCDC which goes through the history and rationale for the changes as well as their latest method.
One adjustment certainly has some logic behind it, the Time of Observation Bias (or TOBS) which adjusts for the fact that in the 1920s, for example, the temperature recorders might measure the temp at 9:00 am and 4:00 pm and count that as the Minimum and the Maximum temperature for the day (the average of the two was then counted as the average temperature.) Newer 24 hour readings show the average temperature doesn’t match those times exactly.
How the old records are adjusted to match the new methods of temperature collection is really the problem. The climate scientists who design the methodology and undertake the actual adjustments to the old records (across thousands of stations) are all global warming believers and allow their own Bias to enter the picture. A little assumption here, a little assumption there, and pretty soon you change a number by 0.7F fairly easily.