Mormon is making reference to the part of the pride cycle in which prosperity leads to pride.
Elder Sterling W. Sill
"…when God adds our goods and blesses us so with education, success and comfort; what do we [do]? We would naturally think that we would respond in kind, that we would be obedient, faithful, humble and grateful. But that is not always what happens. Sometimes in response to an accumulation of blessings, we become arrogant and proud and turn away from the Lord. When God adds peace and prosperity we frequently respond with an increase in our crime wave and a sharp uptrend in our delinquency curve. At least the Prophet said, ’Yea, then is the time that they do harden their hearts, and do forget the Lord their God, and do trample under their feet the Holy One [of Israel]-. And thus we see that except the Lord doth chasten his people with many afflictions, yea, except he doth visit them with death and with terror, and with famine and with all manner of pestilence, they Will not remember him.’ (Helaman 12: 2-3.)
“When we won‘t respond to the Lord’s addition, we force him to resort to subtraction and to take our blessings away from us in our own interest.” (BYU Speeches, March 3, 1964, p. 3)
Neal A. Maxwell
“President Lee… commented on the ’unsteadiness of the hearts of the children of men,’ which has so often meant that at the very time when people are being blessed, the softening of their situation causes them to harden their hearts. (Helaman 12:1-2.)” (Wherefore, Ye Must Press Forward, p. 50)