If it were any type of wine but cava, I would feel very confident in saying that "bonita caída" means "nice viscosity"...but since cava isn't exactly known for that trait, I have my doubts. Here's what I found.

Acto de la cata
- Escuchar atentamente ***la caída del vino en la copa, dará cierta idea de la fluidez o la viscosidad del vino.***
- Mirar bien el vino, puesto que además del color y la tonalidad, se puede observar:
1º. La transparencia al trasluz
2º. Brillos en la superficie
3º. Matices como irisaciones
4º. Lágrimas o patas de vino que pueden ***caer*** por las paredes internas de la copa si se agita un poco.
http://www.turismo-magazine.com/gastronomia/vino/test.php

Bodega: Nekeas,Soc.Coop
Rojo cereza picota.Ribete amoratado muy cubierto. ***Ligera caída de lágrima***. Limpio, intenso y brillante
http://www.ateneu.com/Cercle/Fitxes/NekeasTempCab96.html

Another part of the appearance is its ***viscosity, which runs down the glass when the wine is swirled.*** The slower the wine runs down the glass, the denser the flavor. Therefore, if a glass of red wine is brick color and is slow moving, it is a mature glass of wine.
http://foodfest.neworleans.com/wine.html

"Viscosity" refers to a wine's liquid consistency. A wine's viscosity will may make it appear thin and watery, or may make it appear thick and syrupy.
Wines with high viscosity tend to cling to the side of a wine glass longer, and may leave "tears" or "legs" as bits of the wine begin to drip back down into the glass.
http://www.oregonwines.com/wine101.php?SectionID=8&FaqID=200

Tonight enjoyed a bottle of 1995 Saddleback Cellars Cabernet Sauvignon. This wine made by Nils Venge (of Plumpjack and Groth fame) was a bit hot initially, but when the alcohol blew off the wine packed bright fruit, nice layers and flavors of toffee, black rasberries and dark cherry with a ***nice viscosity*** and beautiful finish of berries that lingered on.
http://radio.weblogs.com/0108247/categories/goodJuice/2002/0...

In the glass, this Grand Ardeche shows a ripe lemon-peel-yellow color with nice viscosity and a clearish rim with little signs of variation
http://www.reviewjournal.com/lvrj_home/2001/Oct-10-Wed-2001/...

The only other thing I can think of is that it has something to do with the sound of the cava going into the glass ("ESCUCHAR atentamente ***la caída del vino en la copa, dará cierta idea de la fluidez o la viscosidad del vino.***), but if there's a term for that, or if it's something that tasters even comment on, I don't know (and don't really have time to look any more).

FWIW...maybe it will help, anyway.

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Note added at 7 hrs 26 mins (2004-11-26 16:15:26 GMT)
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STOP THE PRESSES! Look what I just found! (this one was bugging me). Viscosity *is* apparently used for cava:

CHAMPAGNE / SPARKLING WINES

Cava Juan de Arges Brut

A crafted cava whose viscosity yet subtleness shows us the nobility of its origin. A perfect balance between “macabeo” and “chardonnay”. A dry, light touch of unforgettable elegance

€ 33
http://66.102.9.104/search?q=cache:kaoqM12pbl0J:www.pirestau...