Why is an HDMI signal causing interference with Over-The-Air digital television?
ATSC is an all digital feed.  As such math, can be used to compensate for weak and / or corrupted signals.  This can be done to such an extent that the recovered data may be indistinguishable from the transmitted data.  However, as soon as just enough data is lost to make the math not work... poof... the signal appears to just stop.  Some ATSC decoders still try to display a picture.  These pictures usually look pixilated and jerky.

It may be possible that you are receiving a weak signal.  And that weak signal is susceptible to even minor interference up to the point when the tuner turns the signal into a digital stream.  

A computer is loaded with clocks, signals and devices that generate all kinds of interference.  But, there are all kinds of things you can do to mitigate interference.  Make sure all your antenna cables are clean and that they make good solid contact.  Make sure your computer case is secure and no panels are missing.  Perhaps add a ferrite bead to the HDMI cable.  Re-arrange your TV and computer to see if proximity is effecting reception.  Avoid running noisy cables along side the antenna wire.  Test if keeping cables separate improves reception.  Using a larger antenna or one designed for higher gain.  

You might use an RF amplifier to boost your signal before the ATSC tuner.  In most cases is better to place the amplifier closer to the antenna so you do not amplify noise picked up along the way from the antenna to the ATSC tuner.