Question:
Why are my Acacia simplex plants experiencing the following: the phyllodes (starting from the top, progressing downward) slowly turn yellow, die, and drop; the next highest phyllode is next to experience it.

Is this a sign that it is time to transplant?

About:
My two Acacia simplex plants are in 1 gallon smart pot transplanters; the trees are about 3 feet to 3.5 feet tall.
I (so far) keep them indoors, under fluorescents, on 24/7.

Simplicifolias have a habit of aborting branches, trees in the wild often have more than one large branch instead of a single trunk because the main tip has aborted early in the trees life.

That being said, yellowing and leaf drop from the top down can sometimes be caused by fungal problems at the root zone. Ants invading the potting mix can cause similar problems although the tree will normally go yellow all over if ants have moved in.

The best way to determine if it needs re-potting is to knock the pot off and have a look. If it is rootbound then re-potting might be a good idea.