In reguards to your question about what you're doing wrong, there are a host of answers depending on some circumstances.
Length of exposure, tripod sturdiness, heat waves depending on location...etc.
Post your EXIF Data which will give us some more info to go on and let us know the type of tripod and head you're using.
I know a light weight tripod on an even somewhat windy night can cause all sorts of havoc. A lens only needs to move enough from the shutter flap (are you using the mirro lock up function?) to cause the slightest of blur.
I think from what you're posting, you could shapren it up pretty well in Photochop with a filter>unsharp mask and then play with the settings till you like what you see