The problem is in the logic of your trigger. You need to write code that can handle sets of data instead of single rows at a time.
When you do things like this...
[FruitTrades].[VolTrendPrevD] =
(
SELECT TOP 1 VolTrend FROM FruitTrades
WHERE FID = [Inserted].[FID] AND TradeDate < [Inserted].[TradeDate]
),
...you are not properly handling more than 1 row in your trigger. Also, you have here a top 1 but there is no order by. This means your top 1 is whatever row sql wants to give you.
You could make this trigger work but it would take a complete rewrite to do it correctly.
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