Puzzled by first hiccup - infected, overcarbonation, nucleation?

[tonysbrewery]

Hi everyone,

Intro
First time i register in homebrew forums but usage of this (and other forums) as been religious and a constant for me.
I thank you for the amazing knowledge base this forum provides. It's in the same level as the literature out there when it comes to (home) brewing.

I've had my first hiccup in my homebrew hobby - i've been brewing for a year or so (4 extract and 5 BIAB AG). I would consider myself an expert beginner now!
All the brews have been coming up nicely and it's going really well. However, i had my first big hiccup one of the last couple of brews so i'm looking to understand what might have happened from your valuable experience.

Detail
I had two gushers out of 7 bottles opened:
3 of them were on fridge for 3/4 hours before serving and did not gush.
2 of them were on fridge for more 12-24 hours before serving and were gushers.
1 of them was on the fridge for more than 12-24 hours before serving and did not gush.
1 of them was opened without refrigeration and did not gush.

I degassed one of the ones that gushed and measured gravity. It was the same (maybe one point less) than the FG before bottling (1.012-1.011).
It tastes good but there is a tiny hint of sourness that is unusual in all of the brews i made up to now but this was my first time using Vienna (~20% Grain Bill) and using some Cascade hops i bought last year (they are stored open in freezer in a ziplock bag).
I always VWP and Starsan/Chemsan all equipment before and after brews (bottles and brew equipment except boil kettle!)
What could be the reason for these gushers - true infection, uneven priming sugar mixing in wort before bottling,...?
Looking forward for your knowledge and opinion on this!

My recipe & fermentation details

Grain bill

3.70 kg	Simpsons - Finest Pale Ale Golden Promise		37	2.4	79.6%
850 g	Simpsons - Vienna		36	3.5	18.3%
100 g	Simpsons - Dextrin Malt		31	1.1	2.2%

Hops schedule

8 g	Simcoe		Pellet	13.3	Boil	60 min	15.77	9.3%
12 g	Cascade		Pellet	6.1	Boil	60 min	10.85	14%
10 g	Simcoe		Pellet	13.3	Boil	15 min	9.78	11.6%
15 g	Cascade		Pellet	6.1	Boil	15 min	6.73	17.4%
10 g	Simcoe		Pellet	13.3	Aroma	0 min		11.6%
15 g	Cascade		Pellet	6.1	Aroma	0 min		17.4%
8 g	Amarillo		Pellet	8.9	Dry Hop	4 days		9.3%
8 g	Cascade		Pellet	6.1	Dry Hop	4 days		9.3%

Mash schedule

17 L		Infusion	65.5 °C	60 min
8.4 L		Batch Sparge	20C	15/20 min

Used a 1/3 of a protafloc/whirlfloc tablet and Lalbrew Nottingham High Performance Ale yeast. 3 weeks in primary, 2 weeks in bottle.
OG - 1.060 and FG - 1.012 yielding 6.2% ABV.
I've packaged 13L primed with 88g of table sugar - aiming for 2.4ish carbonation level.

 

[BrewZer]

Assuming you boiled your priming sugar, did you put it in the bottling bucket first and let the beer swirl in from the bottom up, or did you just pour and stir?

Any barely perceptible rings on the gusher bottles (particularly in the neck area)?

Are you sanitizing the bottle caps before capping?

 

[tonysbrewery]

Assuming you boiled your priming sugar, did you put it in the bottling bucket first and let the beer swirl in from the bottom up, or did you just pour and stir?

Any barely perceptible rings on the gusher bottles (particularly in the neck area)?

Are you sanitizing the bottle caps before capping?

Thanks BrewZer!

Yes, i boil the priming sugar in 300/400ml of water.
I do put it in the bottling bucket first and let the syphoning of the fermenter swirl it around. I find this easier. However i can note that the priming solution was still quite warm when i did this and started syphoning into the bottling bucket.

No rings or hop debris whatsoever.

At ambient temperature the bottles are clear/good clarity. I do notice that after refrigeration the bottles seem to be get haze (chill haze or something else?).

I do sanitize caps before bottling by soaking them in starsan/chemsan.

Im puzzled!