1. The population increased
2. The earth was filled with violence and corruption
3. A hybrid race of beings called Nephilim emerge, corresponding with the other two items.
God is not happy about this violence and corruption at all, as it essentially is the full fruition of the Fall we read about in Genesis 3. Therefore, He resolved that He is going to destroy all living beings on the earth, as they are tainted with the corruption which is indicated to be the result of the activities of fallen angels and their Nephilim offspring. But, God doesn't want to totally ditch His original plan, so He separates Noah out. Noah needs an escape plan, and in the remainder of Genesis 6 God gives him one.
Beginning in verse 14, God gives Noah detailed instructions to build a huge boat called an ark. The primary material God wanted Noah to construct the ark from was something called "gopherwood," which has no connection with the burrowing ground squirrel of the same name, but rather is derived from the Hebrew word gofer, which is translated into Greek in the Septuagint as xilon tetrakonon, which is roughly translated as "squared wood." Henry Morris III, in the second volume of our primary text The Book of Beginnings (Dallas: ICR, 2013), notes on page 30 that this "gopher wood" would have had to have been something dense and strong, such as ironwood, but the actual tree is not revealed except to say it must have been an abundant supply. In reading this as well, it has made me rethink my own hypothesis that it possibly was balsa wood too, because although balsa or cork have the buoyancy to float, they may not have been durable enough to maintain structural integrity in the midst of rough weather. At any rate, the "gopher wood" was a tree of God's choosing, and He instructed Noah to use it because of its durability.