One who trusts in his wealth, why will he fall?
R. Yonah #1: He will fall due to his wealth, Midah k'Neged Midah - "Osher Shamur li'V'alav l'Ra'aso" (Koheles 5:12).
R. Yonah: His wealth will fall. It will be lost in a moment, as a punishment for trusting in it - "v'Avad ha'Osher ha'Hu b'Inyan Ra" (Koheles 5:13), "ha'Sa'if Einecha Bo v'Einenu" (below, 23:5).
Malbim: The verse depicts man's affairs as a fruit tree. Its fruit is the fruit of the Nefesh. If he engages in Torah and serves Hashem, "Tzadik ka'Tamar Yifrach"; "it will give its fruit in its time" (Tehilim 92:13, 1:3). External acquisitions such as wealth are like leaves that guard and protect the fruits. So [his Kinyanim] will shield him, lest his needs and income distract him from Torah and Avodas Hashem. One who trusts in his wealth and thinks that it is primary, he will fall like a leaf from the vine. Leaves without the fruit are nothing.
Why does it say "veche'Aleh Tzadikim Yifrachu"?
R. Yonah: Tzadikim trust in Hashem; their salvation will sprout quickly from their afflictions, like a leaf sprouts faster than the fruit. It finishes sprouting quickly. The Ashir will fall suddenly from his honor and serenity, due to his trust in his wealth; the Tzadik will be saved from his affliction quickly, due to his trust in Hashem. This Sefer often discusses the good path and the utter opposite, and the payment that one gets from the good path and its opposite.
Malbim: Tzadikim sprout like a leaf. I.e. they will yield a flower and blossom - the beginning of growing fruits. They will make Kadosh fruits - Chachmah and Da'as. The wealth is like a leaf that surrounds the flower and shields it, so the fruit will last.
