Why did Beethoven deviate in the measure 39 of his last sonata?

[paxxx17]

In the measure 39, the main three-note motif is introduced in the left hand (transposed to E-flat major: Eb, G, D), however, the third note (D), which is generally longer (dotted quarter) and marked sforzando, this time is broken into two eighth notes, with the second one being marked sforzando (and not the first). Compared to the other appearances of this motif, it would've been logical if these two eighth notes were substituted with a quarter note (which is connected to the next beat with a slur).

The measure in question:
https://ibb.co/3cKsdbP


Is there a thematic/structural reason for this deviation, or did Beethoven just write it on a whim?

[lelle]

I'd think of it as the motif getting interrupted by the syncopated note, which is the start of another theme/motif (the winding 16ths that follow)

[quantum]

I'd think of it as the motif getting interrupted by the syncopated note, which is the start of another theme/motif (the winding 16ths that follow)

I concur. 

It is also a very Beethovenian thing to do, using a sf on a weak beat to introduce a new idea, or a surprise, or a contrasting element to the previously established music. 

[paxxx17]

I'd think of it as the motif getting interrupted by the syncopated note, which is the start of another theme/motif (the winding 16ths that follow)

I also thought that was the case, but why was the same not done in the measures 36 and 44 (also in the recapitulation: measures 100 and 108)? Measure 39 is the only instance of this

[brogers70]

I also thought that was the case, but why was the same not done in the measures 36 and 44 (also in the recapitulation: measures 100 and 108)? Measure 39 is the only instance of this

Well, a surprise is not a surprise if it happens all the time, right? I think it's just a little unexpected punch to raise the tension at that point in the movement. One point of creating expectations by doing thing the same way is that you can get some interest and excitement out of breaking the expectation once in a while when you do it differently. Just one more bit of instability inserted into a very agitated and unstable movement.