I recently had a headboard stained and after the topcoat dried it was very sticky to the touch. The same products were used earlier on other furniture with not problem. What causes this? How can I get rid of the sticky residue?

By topcoat, do you mean the topcoat of stain? Or the clear finish that you may have applied after the stain had dried thoroughly? Also, was the stain or finish applied in an area, outside or inside, that has a lot of humidity and/or where there is not much fresh air flow?

Polyacrylic water based topcoat semi-gloss was used. It was applied in an opened garage.

Is the topcoat still sticky? It has been a few days now.

Yes the topcoat is still sticky. The topcoat was applied a few months ago. What would happen if paint thinner was applied to remove the sticky residue?

Water base poly acrylic... either Minwax or Flecto.. both the same material... I have found Lacquer thinner to attack it... really sounds like wax in the base finish... sounds like a total strip & remove procedure.. hit up the minwax website.. maybe they have some insights... sholud dry within 6 hours easily...