dose anyone know why someone would leave coins under the flag pole of a military grave yard?

Do you mean buried beneath the flag pole?

I have been trying to research if this is some kind of common ritual with no success. But when you say "coins", do you mean regular US coinage, or were they coins made by units for award to soldiers by commanders? In any event, so far I think this was just a "personal gesture" made by a fellow comrade .

No direct info but taken from various sources...

1. Also the flagpole doubles as 'the mast' of the museum and there are coins buried beneath the masthead - a custom dating to Roman times.

2. The Fort Hoskins 2 acre parade ground had a 100 foot flagpole at it's center, measuring two feet in diameter at the base. A jar, buried at the base of the flagpole when it was erected, was found in later years containing coins and a company roster. The roster included the names of Officers at the post in charge of Companies F & G of the 4th U.S. Infantry. Lady at the post was Mrs. C.C. Augur, wife of Capt. C. C. Augur and their six children.

3. PROCESS OF DISCOVERY:
Field Notes Archeological Excavations at Fort Clatsop: Paul J. F. Schumacher Archeologist 1961

"...Completing these, the operator removed the old 1908 flagpole stump which had been covered by the concrete platform and marker. The pole stump was 7 ft. deep in the ground beneath the surface. Although legend say coins were dropped in the bottom prior to placing the flagpole, no evidence of coins were found, although the hole was thoroughly cleaned out. There was a mass of greenish tinted clay at the bottom of the hole and struck to the base of the pole, having all the appearance of clay which had been tinted by copper or bronze, but coins would not disintegrate like that in such a short period of years."

4. ...about 600 BC, the people of the Greek city of Ephesus gathered around a big pit in the ground and scattered a group of coins across the bottom of the pit, and then teams of workmen lowered several enormous stone slabs over them. These slabs were the central floor stones of what was to become the Artemision—the Temple of Artemis at Ephesus—known to later ages as one of the Seven Wonders of the World.

The scattering of coins under the floor of the Artemision was an example of one of the most ancient building traditions in the world: the placement of “foundation deposits.” Naval architects will be familiar with a similarly ancient tradition: the insertion of coins into the keels of newly-made ships.

5. On a different morning, James Grant tells us, in the austral summer of 1870...a happy crowd marched in procession to another pit in the ground. They were on the way to lay the foundation stone for Trinity College at the young University of Melbourne.

At the head of the procession were the choristers of St. Peter’s, Eastern Hill, in white surplices: following them came clergy and lay members of the Church Assembly; then Bishop Perry and Dean Macartney in their gowns with other officers of the Diocese; and finally members of the University according to rank, including a sprinkling of doctors of laws in scarlet. Awaiting the procession was an assembly composed very largely of ladies…. Proceedings began with a shortened form of Morning Prayer: St Peter’s Choir with harmonium accompaniment sang Boyce’s anthem, ‘Oh, Where shall wisdom be found’, which the Argus thought ‘very appropriate’ and the Age ‘very pretty’; then Perry descended from the platform and with due ceremony laid the Foundation Stone. This was unmarked, but beneath it, in the usual manner, was a bottle containing the newspapers of the day, the current coins of the realm and a scroll with a Latin commemoration.