The same thing that will cause the "sudden rise in other cryptocurrencies" over the next few months = people seeing Bitcoin is stagnating, learning about other cryptocurrencies, and then converting their Bitcoins into those.

In Litecoin's case it's also because it's one of the very few cryptocurrencies that has been adopted by virtually all exchanges, and it has faster transaction times than Bitcoins (for which transaction times and fees are a big problem currently, after the big recent influx of users/investors).