This phenomenon stems from the fact that the traded currency pairs, there will always be a currency that appreciates in value compared to the other currency in the pair. Ie if you for example buy the EUR / USD, it will do so with the expectation that the euro will appreciate, however, if you sell the EUR / USD, it will do so with the expectation that the euro will depreciate, or which it is equivalent to the US dollar to appreciate.
2) The Forex market is open and operates 24 hours a day. The foreign exchange market is a financial market that operates and is open from Sunday at 5:00 pm EST (10:00 pm GMT), and until Friday at 5:00 pm EST (10:00 pm GMT) .
Thus they are taking a vital times of the sessions of the most important points of world relevance, because in these times the volume of transactions increases and therefore fluctuations in currency pairs are accentuated.