Totally normal operation for the climate control system in the A4. At some point in time you did hit the "Auto" button, which under normal operating conditions (temps above 35 degrees or so [check archives for the exact temperature cutoff]) would turn on the A/C compressor. Chances are, the outside temps in your area have been below that temperature for a while now and the first day above 35 (today), the compressor "magically" comes on.
Ways to avoid this:
1) Don't use the "Auto" button. Adjust temp, fan speed, air output manually.
2) If compressor does come on when you don't want it on, just hit the "*" [snowflake] button to turn off the compressor.
'97 A4 1.8Tqms, Wett 1.0 Bar Chip, ABT Filter, Euro Headlights...