I need to know where these steelhead are that inspire the use of 20-30 pound test leader material! I'm always amazed at how heavy some anglers feel they need to go for a fish species that isn't real toothy and doesn't require a bite tippet. All the steelhead I have ever caught were taken on 8 or 10 pound test Maxima leader material, and that includes specimens of 20 pounds and larger. Of course Maxima tests heavier than is stated on the label, but it's nowhere near twice or three times heavier. I think some flies are harder on leader tippet than salmonids are. Turning over weighted flies, like coneheads, seems like it can weaken a tippet. I'm not certain of that; it could just be that weighted flies are more likely to abrade leader tippet on the river stones.
Not to mention this risks loss of (or serious damage to) the line due to something as simple as getting hung up on the bottom. In most places you really HAVE to be able to break off at the tippet without damaging the rest of your line. Now if you could get a barbless fly to stay in the fish’s mouth the way some of mine manage to stay on a tree branch on the other side of the river - that would be golden!
For the OP - you may be on to something. Unfortunately it could be the tippet. Pure age even if stored under ideal conditions isn’t good by itself. Heat exposure, even 110 in a locked car for a short time may weaken it permanently. Maybe something like that in combination with one of the other scenarios above did it.