The streak is perfectly aligned to the vertical axis. Assuming that it hasn't been rotated, it is likely to be an artifact from imager chip, which may have saturated or overloaded - more charge/pixel than they can linearly process. However I noticed that the color of the streak is pure white, which is hard to understand unless it is caused specifically by the white clouds.
The image of the Earth is slightly saturated, and it can be seen that the bumps in the vertically collapsed image of the earth correlate well with the bumps in the vertically collapsed streak, so while somehow this is likely the cause, the particulars may lie in the processing.
It is important to note that while Orisis' target Bennu has a similar distance from the Sun as the Earth, it's albedo is only about 0.05. The camera is optimized for darker images, so those white clouds may simply be brighter than the design range of the whole system.
Your last link says:
On October 2, 2017, the MapCam instrument on OSIRIS-REx captured the data for a composite image (above) of the Earth and Moon. The spacecraft was approximately 5 million kilometers (3 million miles) from Earth at the time, about 13 times the distance between the Earth and Moon. (Click here to see the geometry of the shot.) Three images (different color wavelengths) were combined and color-corrected to make the composite, and the Moon was “stretched” (brightened) to make it more easily visible.
and according to Spaceflight 101:
MapCam is a medium-range camera. It searches for satellites and outgassing plumes around Bennu, maps the asteroid in color, and provides images to construct topographic maps. Its filter wheel and five-element lens system allows both panchromatic (clear) and wide-band spectral imaging in the blue, green, red, and infrared.
Read more technical information in OCAMS: The OSIRIS-REx Camera Suite
import numpy as np
import matplotlib.pyplot as plt
img = plt.imread('FWC1z.png')[...,:3] # get rid of alpha
earth = img[244:284, 150:250]
streak = 2*img[400:800, 150:250].reshape(40, 10, 100, -1).sum(axis=1) # vert bin by 10
plt.subplot(2, 2, 1)
plt.subplot(2, 2, 3)
for thing in earth.sum(axis=0).T: # plot r, g, b separately
plt.subplot(2, 2, 2)
plt.subplot(2, 2, 4)
for thing in streak.sum(axis=0).T: # plot r, g, b separately
earth_2 = earth[:, 20:80]
plt.subplot(2, 1, 1)
titles = "red", "green", "blue"
for i, (color, title) in enumerate(zip(np.rollaxis(earth_2, 2), titles)):
plt.subplot(6, 1, i+1+3)
print "i = ", i, "title = ", title
for line in color:
plt.text(3, 0.6, title, fontsize=16)