I seem to recall that in <img src="/ubbthreads/images/graemlins/div.gif" alt="" />, the maximum capacity number you see on your character screen is the most your character can carry. The amount your character can hold without being encumbered would be 75% of that number.
As to why it just started being encumbered at a time other than when you picked up a new item, if you had used a Strength potion, food that increases strength, or either of the spells that would increase your strength, when they run out it re-checks if the lowered strength would make you encumbered again. If you had used any of those temporary strength increases a while before that, it would account for it seeming like it's random.
