The problem? The tire and load information placard has been misprinted, and can cause a situation where owners over-inflate the tires. Overloaded tires can potentially fail (read: burst), and can result in a crash.
Toyota spokesperson Bryan Lyons told Automotive News that there isn't a problem with any mechanical part of the Sienna itself, just the sticker. To remedy this situation, the automaker will mail new replacement placards to affected vehicle owners.