Heidegger's Bible Handbook: Philippians: Occasion of the Epistle

2. The money sent to him through Epaphroditus funished the occasion.

Occasion for the writing of this Epistle to the Philippians had been provided, both by the money sent to Paul by the Philippians through Epaphroditus; and by that which he had heard from Epaphroditus concerning the constancy of the Philippians and perfidy of false brethren. Now, just as the Philippians sent money to Saint Paul by Epaphroditus, Philippians 2:25; 4:10, 18; so also Saint Paul sent the same Epaphroditus back to the Philippians with this Epistle, Philippians 2:28.