The answer
St. Ambrose spoke of a custom that was performed only in Milan, where the bishop or priest (who is baptizing) would gird himself and wash the feet of the person being baptized.1371 It is a strange custom since the baptized person emerges from the baptismal font already clean and does not need his feet to be washed at this time. It is possible that the priests wanted to announce to the baptized the fact that the Church will continue washing his feet during his life, even after his baptism.
Fr. Tadros Yacoub Malaty
Notes
1371 See St. Ambrose of Milan, The Sacraments 3.5 (FOTC 44) and On the Mysteries 31, 32 (NPNF II/10).