They usually do this when they see/smell something that they see as a threat but are still confused or unsure of what it it. They usually blow a few times while they try to process it.
If it was just one sharp blow they probably saw you.
No it was a series of blows and I could hear the deer in the thick trees just before the small opening that I'm setup in. I had a bobcat come right under my stand and it never made an indication that it could smell me. A deer's smeller is probably better than bobcats.