Why do some people think John Tyler set up his successor?

Tyler began the annexation of Texas, and his successor, Polk, continued it, leading to war with Mexico, the expansion of slavery in the US, and eventually the Civil War.
Tyler was elected vice president under William Harrison. Harrison died from pneumonia only a month after his term started. Tyler had little support from his own party and failed to win the nomination to run as president on his own.
Tyler was also a strong believer in Manifest Destiny, slavery, and secession. When South Carolina tried to secede during the Nullification Crisis, he supported them. Before the Civil War began, he led the Virginia effort to secede. He also ran and was elected to the Confederate Congress, the only US president to publicly commit treason.
As president, Tyler tried to negotiate annexing Texas. He forced out Daniel Webster as secretary of state and many others in his administration who opposed annexation. He also began a national tour to convince Americans to support annexation. Congress rejected a treaty, and so he was forced to take Texas by congressional resolution.
Polk actually agreed with what Tyler did. He ran for president on the platform of taking Texas, the northern half of Mexico, and Oregon from the British. He negotiated a treaty with Britain, gaining part of Oregon Territory. For Mexico, he deliberately provoked a war. He sent US troops into disputed Mexican territory, where they built a fort. When patrols fought, he sent the false report to Congress that they were "attacked without provocation on American soil."
Both men gained what they wanted: Texas as a slave state and other possible territory for slavery. Neither had the foresight to realize how this would lead to a disastrous civil war and the end of the very slavery they sought to protect and expand.