If you're surprised by the red numbers, then it means that you're a newbie in all of this, if this is true then I think you need to read a lot more about cryptocurrencies and how the market works, you'll find out that this is completely normal.
Nobody can tell you the cause of this dip, could be people cashing out some profits, could be a rumour, perhaps some headlines on the news causing FUD, or just usual volatility. It's hard to know the exact reason behind a dip in the market, but you need to understand that this something that happens all the time, so there's nothing worry about.