Could be anything but given the new round of global uncertainty in the markets again, probably a bit of panic selling or de-risking of portfolios. My only advice is stick to your Stop Loss and monitor your trading system/ rules for a possible re-entry point once the sell off subsides should you feel that VMK is a good share and has been oversold.
Merely means that you adjust the composition of your portfolio where you believe you are exposed to too much risk in any given sector e.g. if you are too exposed to Retail sector and things are looking bleak for this sector, you off load some of your exposure to the Retail sector by selling Retail shares in your portfolio. This is a very simplistic example but hope it explains what I mean't by de-risking.