It essentially means they were moving large amounts of USDT supply around. Presumably there was not enough liquidity on Ethereum and Avalanche mainnets.

Imagine you have multiple games of monopoly in progress but all the monopoly money is managed by a single monopoly money bank.

Two monopoly games (Ethereum and Avalanche) both needed more monopoly money for the game to go on smoothly, so the banker transferred some money from their main bank (Tron Network) to these other games that are in progress. The total amount of monopoly money in circulation did not change, but the amount of money available in each in-progress game has changed.