First, deflation is a general
fall of prices (or conversely, a general appreciation of the value of money in terms of goods and services). Whether there is deflation or inflation and their size depend amongst others
on the quantity of money relative to the level of economic activity (the output of goods and services). When the quantity of money rises more quickly than output there are inflationary pressures, when it rises more slowly deflationary pressures. The quantity of bitcoin is unconnected to the level of economic activity and from I know of it it gets harder to make them and as there are more of them. Hence, should it be adopted as currency, it's probable that the quantity of money would not keep up with the size of a growing economy (this was also a big problem with gold in the 19th century). Hence it could be deflationary.
It's not so much an asset that's deflationary, it's that if it's used as money and the supply doesn't keep up with economic activity that it could be deflationary.
All of which is very academic, because the much bigger practical problem with bitcoin is that its value seesaws like crazy.