Most problems are caused by not fully preparing a decks surfaces, choosing the wrong products or local weather conditions during and after application.
I recently stripped a redwood deck that was tacky after the homeowner applied a deck stain. It was 3-4 weeks after the old stain was applied before I stripped it off and it was very soft and a little tacky. The stain was a water based acrylic purchased from the local home improvement center. Technically, it was garbage.
One possibility could be caused by not removing the the old deck stain. In most cases I recommend a good washing and sanding to bring up new sound wood. This will insure that no weird reactions with the old stain will take place.
Another possibility is the use of Linseed Oil as a sealer without proper thinning with turpentine. Linseed Oil can remain soft and a little tacky for weeks and maybe months in a humid environment.
Wet wood prior to sealing can cause all kinds of problems. Plus high humidity after applying a stain can drastically slowdown curing.
Without knowing what product(s) you used and how the deck was prepared, it is a little difficult to say what is the exact cause.