RPGA adventures are linearly structured due to the necessity of completing the adventure in a 4-hour time slot. Here are a few things any DM can do to break up the monotony and railroad:
1. Prep and playtest the adventure beforehand. If the DM is really prepared and has timed out each section of the adventure, s/he can usually find some time during skill challenges or roleplaying interactions to allow some creative and experimental actions by the PCs (or just some immersive RP).
2. On timing out the adventures -- it really helps to put a "time stamp" on each section or encounter of the adventure once it's prepped/playtested (encounter 1 = 30 mins, encounter 2 = 15 mins, encounter 3 = 1 hour, etc.). Then, keep one eye on your cell phone or watch to make sure you're within approximate time limits for each encounter. For example, in CORE1-3 (Sense of Wonder) there is a huge running battle at the end of the adventure, but only one other combat encounter, and that's right at the beginning. So you can plan ahead and know there's about a 1.5 hour stretch in the middle of the module for skill challenges and role-playing.
3. When adjudicating skill challenges, use your DM Empowerment to broaden the scope of the challenge, and allow "off-skill" usage and creative solutions at +5 DC so everyone who wants to contribute, can contribute. But in order to accomplish this, you need to understand what the setup and goal of each skill challenge is. Again, I cannot stress enough how a pre-gameday playtest session increases one's knowledge of how to run a given adventure within the 4 hour time slot.
4. Understand that a certain degree of linearity is essentially required and expected when running RPGA adventures. The players know it too, if they are at all experienced in the RPGA. They want to finish the adventure, succeed to the utmost, and get the loot and magical goodies at the end... so they have an incentive to follow the plot hooks you present.
I'm not an RPGA rep or anything like that, but I've run many successful tables so I hope this advice is helpful. You can't really go for that idealized sandbox-type gaming experience in RPGA adventures, but they can be great fun and very rewarding to run if one is properly prepared.