Transitioning from the seriousness of Y"K to the simcha of Sukkos is apt to give one spiritual whiplash, and I am not sure that having to think about parshas Ha'azinu in the middle makes it any easier. Not to mention that there is not much time to prepare the parsha with all else that is going on.
וַיִּשְׁמַ֤ן יְשֻׁרוּן֙ וַיִּבְעָ֔ט שָׁמַ֖נְתָּ עָבִ֣יתָ כָּשִׂ֑יתָ וַיִּטֹּשׁ֙ אלו-ק עָשָׂ֔הוּ וַיְנַבֵּ֖ל צ֥וּר יְשֻׁעָתֽוֹ (32:15)
First the simple pshat and then a really sharp Sefas Emes.
The appellation Yeshurun is not too common. It appears only 4 times in Tanach, and of the 4, interestingly, three appear at the end of Sefer Devarim. Why does the pasuk use such an uncommon term here? Seforno (see Netziv as well) explains that it refers specifically to קהל תופשי התורה ובעלי העיון, the spiritual elite, what we might call the yeshiva community (whether the yeshuiva is YU or Lakewood doesn't matter). The pasuk is warning that not only can regular Joe Yisrael's religious commitment be damaged by too much enjoyment of worldly pleasures, but even those on the top rung of the community religiously, those who we least expect it to happen to and who we might think are immune from such things, even they will quickly slide down the ladder if they indulge too much in pleasure. Ad kan what is a fairly straightforward message.
Sefas Emes (5637), however, says we are not talking about growing fat on the pleasures of olam ha'zeh here. Peshita that if that's the case, one will drift away. Yeshurun is all about those who are doing the right thing, those who are growing fat on Torah and mitzvos. It's days before Sukkos and everyone is buying lulav and esrog, everyone is putting up their sukkah, people are learning daf yomi, tomchei shabbos is delivering food packages for Y"T. What the Torah is telling us is that sometimes a person can do all of the above, and still miss the boat. How? Because a person can get so caught up in what THEY are doing for Y"T, for chessed, for Torah, that they forget that someone else is really directing the show. וַיִּטֹּשׁ֙ אלו-ק עָשָׂ֔הוּ, they forget that G-D is the one doing it all. It is possible to be fully immersed in religion at the expense of having a relationship with G-d.
Another interesting twist in the pasuk is that it switches from third person וַיִּשְׁמַ֤ן ... וַיִּבְעָ֔ט to second person שָׁמַ֖נְתָּ עָבִ֣יתָ כָּשִׂ֑יתָ. Ibn Ezra learns that the third person is the pasuk's description of what is happening; the second person voice is the person speaking to himself. Every person at some point stumbles, but even if there is a וַיִּשְׁמַ֤ן יְשֻׁרוּן֙ , so long as a person at some point wakes up and says to himself, שָׁמַ֖נְתָּ עָבִ֣יתָ כָּשִׂ֑יתָ, "You are losing your focus and going in the wrong direction," they will recover. When Ibn Ezra comments שמנת – ולא חשב לאמר לנפשו: שמנת עבית כשית, I'm not sure this is exactly what he means, but the way I read it is the וַיִּבְעָ֔ט is modifying those next words of שָׁמַ֖נְתָּ עָבִ֣יתָ כָּשִׂ֑יתָ. When a person turns off and rejects the message of that inner voice, then they are in real trouble.