What do they gain by sabotaging Gus's business?
What do you know about the relationship between Gus Fring and the Salamancas? I'm specifically pointing out a flashback scene where Fring has his first meeting with Don Eladio. I don't remember if this happens before or after S04E06, I suspect before but I'm avoiding spoilers just in case.
They're working for the same cartel, but does that mean that they necessarily share a cooperative spirit between them?
More relevantly, the cartel engages in smuggling things across the border. This is a liability, but it's easier to cook meth in Mexico than it is to cook it in the US, relating to the cartel having more control and freedom in Mexico. This is the part of the business that the Salamancas are engaged in. Getting caught crossing the border is part of it.
Fring, however, seems to not get caught at the border, his sales figures are great, ... clearly, he's doing something well. We know he's cooking meth locally and thus avoiding the need to risk smuggling it across the border, but the cartel and the Salamancas don't need to be actively aware of that in order to see his "taxes" to the cartel and his accompanying sales figures.
This is making the Salamancas look bad in comparison, therefore losing out on Don Eladio's favor. And what trait does the Salamanca family live by? Violence and retaliation.