I can't speak for all of eastern North America, but climate likely played a significant role. For my example, I'll focus on Cahokia, the largest pre-Columbian settlement north of Mexico. Cahokia rapidly grew into the largest polity in the American Bottom near modern St Louis around AD 1050, enjoying a relatively warm and wet climate. This is important because the people relied considerably on maize agriculture. Cahokia had a climax of perhaps a hundred years or so before significant decline began and the region became extensively depopulated. Explaining this has been an active field of research for awhile now.

Environmentally, some early research suggested deforestation for maize fields and fuel consumption led to increased erosion and flooded fields. The pollen data don't back this up - the region had been cleared by at least AD 450 or so. Later work (particularly that of Larry Benson) using tree ring drought records demonstrated that Cahokia developed during a wet period before multidecadal droughts set in.

More recently, work using oxygen isotopes on carbonates gives us a better handle on the climate. These data show a change in the precipitation regime coincident with Cahokia's collapse. Precipitation across the Midwest is generally derived from the northern Pacific or from the Gulf. Importantly, the isotopic signatures of these are distinct. While Cahokia was booming, a lot of the precipitation was coming from the Gulf and this typically occurs in the summer. This is good for maize agriculture. As the climate changed, precipitation started coming from the Pacific and in the winter instead. Simultaneously, the same sediment cores yielding this carbonate data also show significant flooding of the area around AD 1150 and periodic flooding afterwards. Taken together, Cahokia developed in a favorable climate in the AD 1000s and early 1100s before summer rains dried up and, funny enough, devastating flooding occurred. This clearly threatened the maize harvests the people relied on. Cahokia was also likely dominated by an institutionalized priesthood. These events arguably delegitimized the ruling class.

Now, Cahokia is interesting by itself, but it has broader importance because it is really where the constellation of traits that mark the Mississippian period begin. Recent research suggests migrants from Cahokia, possibly fleeing the worsening climate, catalyzed cultural changes in the Ohio Valley.