There's more to it than photos - though that's probably the cause in the example you gave.
Way back, something I did forced the horizontal scrolling thing.
Then I edited my post, and it was OK again.
Sadly, I have no idea now what exactly I had done. But it was obvious to me at the time how I could edit my post to make it OK again.
If I remember in the next 12 hours, I will post again.
REMEMBERED!
Inaverypoorly-judgedmomentofhumourIhadtypedacontinuousstreamofwordswithnospacesandnopunctuationthatwentonformanymanywordsandwasreallyhardtodobecausemyspacebarthumbworksalmostautomaticallyasIcantouchtypeandtheendresultwasthattheforumdisplayerthoughtitwasallonewordandsodemandedlotsofhorizontalscrollWhenIsawtheeffectofwhatIhaddoneIwentbackintoeditandsplitthelongwordwhichimmediatelystoppedthehorizontalscrolling.
I will get back in and do the same to this post - soon.