It could be a genuine reason for sure but cancelling by text as soon as we end the call seems fishy to me.

I only book new Clients by phonecall, so when they think it's ok to send a 1 line text to then cancel, I think it's ignorant.
 What if I never heard or saw it because I'm busy getting ready and in the shower or even later, driving up the motorway to them? 

If they cancel immediately, I never respond but will make a note of them and send them to spam.
They languish there forever and I have forgotten them five minutes later   

Those who immediately cancel after making the booking get no second chance with me. They don't have my full address at that stage anyway so no worries for me.

No way for sure of knowing their motives but as others said they could have booked more than one and cancelled the "unlucky one" but whatever their reasons I log them as timewasters and ignore any more calls or texts from those numbers.

Right, this. I always figure they're hedging their bets & their favorite/cheaper/closer/take your pick answered them.

The next time they call (& they always do) I say, "Now lets see. I was supposed to see you on X day at Y time, but you never showed up, nor did you call. Care to explain that?" *best death voice*

Much stuttering ensues & I grill them on why I should believe them this time. I also remind them that "this is a large industry, but a small community & well....*pause*...word DOES get around."

Here stateside we have an app called Mr Number you can DL that's useful; you can make notes under the 'general spam' area & it's a real lifesaver.