I'm having the same problem with several plants this year and I think it might be the cold and dry spring. Can't find any other reason.

Worse case is that your aster has a virus or a parasitic fungus that causes the leaves to distort as it grows. It will also cause distortion of the flowers too. There is no cure other than to get rid of the whole plant..bag it and throw it into the garbage. Furthermore consider not planting anything in that spot until next year. Also, if you do dig it out, soak the working end of your garden tool in a one part bleach to nine parts water solution for a couple of minutes then rinse with water in order to avoid contamination elsewhere in your garden and don't forget to wash your hands too after touching the plant to also avoid transmission to other plants.

could be thrip damage as the leaves were forming. they suck sap and are very small and difficult to see.

It is thrip that is a major vector of the viruses that cause leaf wilt. It carries the virus during its whole life cycle so all it takes is one carrier thrip to infect a plant when feeding. That is why one sees a whole plant going down without seeing a heavy thrip infestation as one is accustomed to. The virus is called a Tospovirus and it has acquired the ability to infect over 800 plant species and the number is growing. Vegetable plants infected are watermelon, peanut, pepper, zucchini to name a few. It is also infecting flowering plants now such as the well known impatien infestation here in the USA which is now causing the flowering plant greenhouse industry great problems in that the virus is modifying to infect other flowering plants the begonia being the most recent one. This virus is a cousin to a virus that infects humans and goes by the name of Hantavirus.

Thanks for your comments