So far today the S&P/ASX 200 (Index: ^AXJO) (ASX: XJO) is just managing to keep its head above water and in afternoon trade is up 0.1% to 5,333 points.

Acting as a major drag on the market today have been four shares in particular. Each has dropped significant lower, here’s why…

Aconex Ltd (ASX: ACX) shares have plunged for a fourth trading session in a row. This time its shares have dropped 7% to $4.50. The only news out of the company this week has been the planned retirement of its CFO next year. Although I would class this as a non-event, the market appears to see things differently. In my opinion the fall in Aconex’s share price makes it even more appealing as a long-term buy and hold investment.

Fortescue Metals Group Limited (ASX: FMG) shares have dropped 3% to $5.90 after iron ore prices plunged 6.5% overnight. Reports in the Fairfax Media indicate that even producers think these high prices are unsustainable. As a result I wouldn’t be surprised to see prices continue to tumble over the next few months. Now might be a great time to take a profit and move out of the iron ore miners in my opinion.

iCar Asia Ltd (ASX: ICQ) shares are down 13% to 20 cents after it emerged that two non-executive directors from Carsales.Com Ltd (ASX: CAR) have resigned from its board. This could be a sign that Carsales has lost patience with the underperforming company and plans to dump its 16.5% stake. Alternatively, a bullish view could be that it plans to launch a takeover in a bid to turn around its fortunes.

Mayne Pharma Group Ltd (ASX: MYX) shares have fallen 5% to $1.56 despite no news out of the pharmaceutical company. The company is currently being investigated by the US Department of Justice regarding price-fixing. Although it does not expect any penalties imposed to be material to earnings, until the matter is resolved I suspect its shares could remain volatile. I still see the company as a great buy and hold investment and would view the current weakness as a buying opportunity.

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Motley Fool contributor James Mickleboro has no position in any stocks mentioned. The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.