Doesn't makes any sense to me, why they will want to sabotage their own drivers?
It started in Istanbul last year, when Webber and Vettel collided. Everyone blamed Vettel, but Helmut Marko was quick to point the finger at Webber. To be fair, the HANS device prevents a driver from turning his head from side to side (it's to prevent neck injuries), but even if it did not, the high sides of the cockpit and the narrow range of the wing mirrors would have made it impossible for Webber to see Vettel anyway.
Then Red Bull switched the front wings on the cars at Silverstone when Vettel broke his. Their reasoning for it was understandable, given that Vettel had clearly been faster all weekend, but Webber ran to the media and started crying wolf. And ever since then, everyone is quick to believe that Red Bull are favouring Vettel and deliberately sabotaging Webber to prevent him from challenging their preferred driver. Some of the theories are really quite elaborate, like the team calling Webber in early at Abu Dhabi last year to force Alonso to respond (since Webber was Alonso's most immediate challenger for the title) and take Ferrari out of play so that Vettel could win the championship.
The truth of it is that Red Bull really screwed up in 2009 when they didn't favour one driver. By rights, Sebastian Vettel should have been the 2009 World Champion - but because Red Bull let their drivers race one another and, more importantly, take points away from one another, in the middle of the season when Brawn and Jenson Button were vulnerable (because of a bad upgrade and a delayed reaction to it; they initially wrote off their poor performances as being a result of cold-weather racing in a car that performed best at hot races), Button was able to build up enough of a points lead that he did not have to push too hard at the end of the season (most people think Button simply gave up and cruised to the Championship, but in reality, Brawn ran out of money trying to fix their bad upgrade).
There are elements in the team that show favouritism. But there is no massive conspiracy - Webber has been noticeably slower than Vettel all year. He didn't start to come back at Vettel until Pirelli changed the make-up of the tyres to last a little longer (since they felt regular three- and four-stop races were taking things too far). Nevertheless, the conspiracy theorists in their tin foil hats believe that Red Bull have been subtly sabotaging Webber just enough to stop him from challenging Vettel, but not so much that he is exposed to McLaren and Ferrari.