Although I haven't tasted the Bockfliess Gruner, I do know that in some light whites (and a few light reds) winemakers leave behind or add some CO2 or carbon dioxide, which creates bubbles.
Bubbles increase the perception of acidity. Ever had a flat Champagne or flat soda? They seem less crisp and more sweet than the same beverage with bubbles.
Why would a winemaker want to increase the perception of acidity in their wine?
Well, as you mentioned, 2003 was a hot vintage in Austria (most of Europe baked that year). This heat means that the grapes got really, really ripe. And, as grapes ripen, they lose acidity. In toasty vintages like 2003, grapes can come into the winery with low levels of acidity.
Wines with low acidity are generally not very refreshing or crisp; they can be downright flabby. Low acidity would be especially bad for Gruner, which is known for its crisp acidity.
Was CO2 intentionelly left in the 2003 Bockfliess Gruner? I don't know. I have emailed a couple of retailers who sell the Bockfliess Gruner to see if they have any experience with this specific wine. We'll see if they reply.
Could the spritz have been caused by residual sugar and a secondary fermentation in the bottle? Probably not, unless the wine tasted less-than-good.
How was the wine?