it may be becos of rough handling

Could be a spine development defect (something in the notochord) or caudal intervertebral disc degeneration?

But if they were adult at the time, then it may simply be rough handling or fighting within the cage. We have seen this on occasion with mid-old age mice. 

There are many possibilities. Spine defect, degeneration, fighting or handling problems. I also observed this before, both in young and old mice.

However there are some interesting mutations:

http://mousemutant.jax.org/articles/mmrmutantbolt.html

"A new dominant skeletal mutation named lightning bolt tail (Bolt) has been found at The Jackson Laboratory. Mice carrying the Bolt mutation have kinked tails and shortened spines. This new mutation has been mapped to Chromosome 11."

Please  have a look at it and let me know what do you think.

I also suggest you to keep the mouse tail and do a skeletal stain (I can check if I steel have the protocol here).

I've seen it occasionally if repeated tail bleeds are performed with poor handling procedures. In that situation the kink isn't very pronounced. Depending on age and sex it may also be due to aggression between animals - tail rattling and biting perhaps, but this would be unusual. 

If this anomalie is due to 'poor handling' or genotyping  then I suggest you go back and redo your animal handling license training or get the animal tecs to show you how to handle everything from young new born pups to adult mice. All mice need their tail intact...its an extension of their spinal column

I doubt that their kinky tail phenotype would come on during adulthood. It is most likely from an injury.  Do you have other mice of the same genotype (at the same age)? If so, do all their tails look normal?

Most likely from injuries.

I also had one, I really think it was first handling mouse and also few experience on it.