- The main engine throttles down during Max-Q: I would love this explanation because I think it is a great, visual demonstration of the concept - but I'm afraid that the main engines blue flame is too dim compared with the SRBs to have such a big effect.
- The angle of the shuttle trajectory changes: This makes the most sense, the change in angle makes the shuttle appear to move slower across the sky (even though it is moving faster), leaving more time to illuminate the film (or ccd) and a resulting brighter spot. An angle change could also make the flame look brighter or dimmer.
Any other ideas? Leave them in the comments.
The photo is by James Vernacotola and it won an honorable mention in the 2012 National Geographic photo contest.
To make my question a little clearer I've added this photo showing the confusing increase in brightness.
Edit: Not even 12 hours after posting we have an answer, thanks to a suggestion by Luke and a helpful link from Eli (see comments). The height of the shuttle at the point of extra brightness was about 20,000 ft. which puts it right in the middle of tropical cirrus cloud altitude resulting in a bright spot even though the actual rocket was not any brighter (likely dimmer).