There can be plenty of reasons to wear many-sizes-ago clothing.
Sibling-sorting mixup.
Bottom of drawer stow-away.
Wishing child to forever remain toddler size.
Deemed too cute to pass-along.
Won't ever find anything as soft.
Child's most-favorite article of clothing.
(some of those don't apply to DJ)
In this case, I think the PJ pants may have been borderline too-short last winter. Combo that with his recent 'growing-like-a-weed' phase, and wallah...