It is when urine travels the wrong way up the ureters and exerts pressure on the kidneys.
Hence the use of the phrase, “tends to”.
I had this when I took one particular anti-histamine. Checked the side effects and it was listed right there. Never took that one again.
Whoa! I didn’t even know that was possible. Egads.
When I hear ‘reflux’ I think ‘acid reflux’, not ‘urinary reflux’.
Can that happen to both sexes?
Yes.
99+% of the time when my patients tell me their kidneys hurt, there is no problem with their kidneys. It’s usually musculo-skeletal back pain. On occasion it’s ureteral pain from a stone. Your average “kidney” stone doesn’t cause pain until it leaves the kidney and enters into the ureter, but there are common exceptions to this.
Once in a while, they get it right, and it’s due to actual kidney irritation from infection or stone, or similar (such as a completely blocked ureter.)
What causes it? (other than a few rather obscure ways I’m aware of related to spinal-cord injury)
That’s why Doan’s Kidney Pills were so popular - lots of people were sure they were having “kidney pain”, and the anti-inflammatory in the “kidney pills” worked to alleviate the musculo-skeletal aches and pains they were having. A win-win! 
That is, until the feds nixed the “kidney” pills thing.
The term most often used for this is ‘vesicoureteral reflux’. Try the NIH site. The Wiki one doesn’t look too helpful to me.
Bottom line is that kids can be born with it, whereas adults tend to get it secondarily from other problems in the urinary system such as prostate enlargement in men, bladder infections in both sexes, and as a complication of diabetes (“neurogenic bladder”). All of these things have in common an increase in pressure in the bladder which leads to a backing up of the urine (into the ureters and even kidneys, with the ureters being the tubes that connect the kidneys to the bladder).
You do realize that is a kidney stone the size of Delaware, don’t you?
That must be why kidney stone sufferers are always bent Dover in pain.
Cleverness fail. 
I appreciate your candor.
Okay, do that with Rhode Island.
Only Providence could create such a stone!
May I mention that many people consider back pain to be an annoyance and just something that comes with age. That’s what my husband and I thought with his back pain in his kidney region. He was Dx with renal cell cancer after about 3 mos. of it.
Delaware!?! No no no no. Texas, my friend. Before they took it to the museum I set it beside a penny and snapped a photo. Alas, gone when my dog ate my cell phone. But as I recall it was about 1/2 the size of Lincoln’s noodle. Even he was agog - just kept staring at it, speechless. Then two big fellas came and carted it off.
Ahhh, I see. Thanks. (I must have missed this post in my initial rush to sarcasm ;))
Just to give you all closure, because I know you’ve been anxiously awaiting the status of my kidneys- the pain must have just been a foreshadowing of one of the worst episodes of back pain I’ve ever experienced. I have some smushed disks or something in my lower back, and once a year or so, it gets really bad, and I usually end up close to overdosing on ibuprofen and then in the ER for a shot of morphine, and that fixes me up for a good long time. This time I didn’t have to go to the ER- the ibuprofen seems to have worked. Whew, it was painful, and I’m glad that it seems to be over for now. But at least my kidneys are alive and well.
OTC treatment…and kidneys that aren’t attacking you. Seems like a win to me! 
Good news.
-D/a