Breloom is Grass-Fighting type.
The weather condition sandstorm only affects Rock-, Ground-, or Steel-type Pokemon.
Cacnea is Grass-type, and I see your theory.
However, this Cacnea obviously had the Sand Veil ability. The Sand Veil ability makes it immune to sandstorm, and it also increases it's evasion rate in a sandstorm.
So, there's your problem.
Hope I helped. :)
Source: Second Paragraph