You know that's actually where the word comes from?
Tun is the English spelling of tonne (plural tonneaux), originally a 900 litre wine barrel*, later a general term for any similar container. Since the empty tonne weighs about 100 kg and the wine weighs about 900 kg, "tonne" got chosen as the name for a megagram.
Further factoid: "shipping tonnes" weren't a weight at all, but a volume: the number of 900 litre barrels you could fit on board. Ships are now rated in TFCEs.
*A barrel half the size of a tonne is a puncheon, a third is a hogshead, a quarter is a quarter cask or barrique and an eighth is an octave.