How did I blow a 12ax7 so quickly?

Hey folks. So I acquired a Mark V 25 head in mid-June, and one of the biggest selling features was the built-in headphone jack and attenuator. I am a bedroom player who aspires to gig in the not-too-distant future. For practicing, I often play along with youtube backing tracks. My old practice amp had an aux in, but lacking that on the Mark V, I went out and bought a splitter. Two males to a female out. I was plugging into the headphone out and my computer speaker out, and running both signals to a single pair of headphones. I was using the splitter for about a week with no issues. Then one day I turned my amp on and there was no sound coming out. I took it to the shop and they told me they had to replace the "v5-12ax7." So my question is, was the tube going so quickly in any way related to using the splitter. I'm thinking no, but I want to be sure these are in no way related. Is it safe to keep using the splitter or do I need to invest in a small mixer?

I doubt the splitter is related. I have blown tubes in 3 months, 6 months, 1 year and have one amp that is 10 years old with the stock tubes. It's a bit of a crap shoot. Kind of like light bulbs.

I've had a new tube last less than five minutes more than once.
It's just the nature of tubes. Basically they are similar to light bulbs but more complex. Some are going to last longer than others.
I have other tubes that have worked over forty years.

As other have said, tube life is a crap shoot. The rectifier in my brand new mark v blew within an hour. Way she goes boys.