Like every fiat currency today, bitcoin has assumed the position of a currency though typified as a crypto-currency. But in reality, it is obvious that the value of any country's currency depends to a large extent on her economic dynamics and activities (demand, supply, external reserve, level of production, etc.) . By that i mean, the strength of an economy determines the strength or purchasing power of her currency against those of others.

On this basis what can we allude to be the cause of the recent decline in value of bitcoin?

This is because, the United States, China, Europe etc experienced mild growth in the 2018 fiscal year as such one would have expected an improvement in the value of bitcoin just as the value of those other countries have either stabilized or improved.