
I believe it is related to the dust, change in pollen, grass etc Summertime brings so much along with it.
I started my hayfever tablets last week and that usually is about the sametime I hear people saying their dogs are reverse sneezing.
Of course you would need to speak to your vet but Piriton or an alternative remedy for pollen I think works.
I have been reading it on a few on the boards I am on just now.