Not because they expect them to deflate due to scarcity, but because they expect the probability that the expected deflation will actually occur will increase. My argument only applies to expected (in the sense of generally agreed to be a near certainty) deflation.

Imagine if there was universal agreement that Bitcoins would be worth $1,000 each next year. The price would rapidly shoot up to near $1,000 now, nobody would sell for much less and nobody would buy for much more. Then there would no longer be any need to hold them to next year to get the deflated value.

The reason the price is going up is not because they will be worth $1,000 in ten years and it's getting closer to then. It's because people are becoming more convinced that the value might actually be $1,000 in ten years. That is, it's not deflation that's raising its value. It's that people are realizing they might actually deflate.