when brokers suspect that volatility is about to spike, they reduce leverage so as to prevent your trading account from going into negative should the market move against your position swiftly.
When leverage is reduced, the broker requires you deposit more margin in order to open a trade, thus shifting much of the risk from themselves to you.
Every broker I have traded with, has periods when they reduce the leverage, how ever some brokers do it more often than others.
Brokers who reduce leverage too frequently may not be as robust (financially/balncesheet wise) as those who dont do so frequently.
For example, today I tried to trade the EUR/USD at the start of the Pre-NewYork session (1 PM) but i couldnt open a position because the broker required more margin so I had to miss out on the trade which would have been profitable.
This is why I always trade with more than one broker, so that when one reduces leverage, I can quickly go to the other broker to open the trade.