The guides obviously were well aware of the hazard, and handled the crossing with that in mind. I couldn't tell from the video (and my Spanish is poor), but I'll bet they had someone posted as a lookout in a spot they could see upstream and yell a warning if a big one was coming down.
In organized avalanche rescue/recovery, if there is even a slight chance of another slide coming down the same path, it is standard practice to post a safety lookout. An escape route is also clearly defined. Everyone working on the debris pile knows if the lookout yells, instantly drop what you are doing and run like hell on the escape route. (Note that if there is a significant chance of another slide, recovery ops probably will not occur until control work is done to eliminate the additional hazard.)
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"Toto, I've a feeling we're not in Kansas any more."
-Dorothy, in The Wizard of Oz