I agree with the above.
The fact that it broke at the knot means that it was either the knot that was used or how it was tied. Ask you self why it didn't break at the hook knot which should be the weakest knot since this knot is has much more stress and motion with slippage around the metal hook eye, and abrasion during the fight. The tippet knot has only longitudinal stress. It should hold when tied correctly.
Lubrication is the key. But I also think technology can help with these knots on thin tippets. UV Knot Sense
is suggested. It will reinforce the knot and prevent slippage, which I think is what caused your failures. I think it is a good possibility that the knot was not completely "set".
The other thing I would say is that good 6X tippet is pretty strong at 3 lbs. Try lifting 2 lbs of dead weight with your rod and it is difficult. 3 lbs is stronger than the strongest tippet when I started fishing. 5X Nylorfi was about 2.5 lbs.
I think there was a sudden jerk on the leader, the knot slipped and broke. The best way to prevent this is with a softer tip fly rod. Failing that, "shock gum
", basically a high tech rubber band between the fly line and leader will help.
Finally, do a "shock test" on the knot after you tie it. Give it a tug and see if it breaks. Better to have to retie than to lose a fish.
1. Use a good knot.
2. Lubricate and pull the knot tight so that it sets.
3. Use UV knot sense.
4. Test the knot after tying.
5. Use Shock gum or a softer tip fly rod.
You didn't say what knot you used. I'm curious to know.