Rarefaction.single plots

I’m hoping someone here can explain something odd I’m seeing in my rarefaction data. I’m not sure if it’s a bug or if it’s just something about the data.
Previously when ever I’ve done rarefaction plots I’ve gotten nice smooth (or relatively smooth curves).
But my current data set is showing the start of a reasonable curve and then a sudden decrease and then the rest of the curve looks nice and smooth.
(so I’m getting a peak in the middle of the curve the curve looks sort of like:
…X …

(If the above graphic makes any sense)

Is this my data that’s just being strange (which I’d believe since we didn’t get the number of reads we had expected)? or is it some sort of bug?
Is there a biological reason that might cause a rarefaction data to increase steadily then suddenly drop? (I’ve never seen something like this before)

The section of the rarefaction that’s giving me the weird peak is
3400 480.6385
3500 480.5859
3600 483.6719
3700 482.6865
3800 481.8964
3900 482.4572
4000 482.3346
4100 481.6426
4200 478.3655
4300 471.9467
4400 467.7978
4500 455.5024
4600 444.1183
4700 433.8459
4800 431.2077

Thanks to anyone who helps explain this.

What variable are you rarefying? Can you send your rabund/shared file and how you’re running the command to mothur.bugs@gmail.com? Also, please reference the url of this posting