I am also confuse on how ailment are applied.
Especially how much maximum stacks can be applied.
What are the requirements (nodes, gear)?
Thanks

I think it depends what damage you have on your weapon.
I noticed once I put a "Adds lighting damage to spells" gem into my weapon my spells now add shocking.

The number of ailment stacks can bee seen in the character sheet, standard is 10 you can get more in the passive tree up to 30.

The Most likely reason this happens to you is because of Immortal Offering Node.

for example:
Skill 1 deals 90% Fire, 10% Frost damage.
Skill 2 deals 90% Lightning, 10% Rend damage.

you kill a mob with Skill 1 first. You get Immortal Offering stacks, 5x Fire, 5x Frost.

Now Skill 2 deals 60% Lightning, 5% Rend, 17.5% Fire and 17.5% Frost Damage.

Which means that Skill 2 will now apply Lightning and Fire ailments (the highest damage types)

Basically Immortal Offering takes the TOTAL damage of a skill, and adds 25% MORE damage of the element you have stacks of.

It can be confusing. Immortal Offering is INCREDIBLY strong (150% multiplier with 6 ailments) BUT you have to keep this mechanic in mind - It screws with your ailment applications. keep that in mind:)

*Note: Percentages are not exact, they're just an example.

what you write sounds absolute logic now...
but... shouldn´t immortal offering increase ALL dmg and not only the dmg type it is granted from?
sounds buggy to me?

It does increase ALL damage but it adds it as another element.

for clarity, If your Skill deals 800 Fire and 200 Frost damage, and you get 5 stacks of Immortal Offering from Lightning,
the same spell will now deal:

800 Fire, 200 Frost and 250 Lightning Damage. I don't think it's bugged, But it's a bit unclear in its description.

regardless, It's incredibly strong since you can double your damage output with just 4 different ailments... downside is It keeps messing with what ailments you actually apply :p

p.s. It even increases ailment damage. If your BURN ailment ticks for 10k fire damage and you have 5 stacks of Immortal Offering from Frost, Fire, Lightning and Shadow, it will now tick for:
12.5k fire
2.5k Frost
2.5k Lightning
2.5k Shadow

I hope that makes sense. It can be a bit of a hassle to work around, but Its easily worth it.

ok that makes sense^^
but how did you figure that out?
do you have a kind of dps meter?

No, I had the same problem as you and thought Immortal Offering might be what was causing it so I started doing some experiments and this was the conclusion.

If you check your skills in the detailed screen you can see exactly what damage types they are dealing, how much etc, and it changes depending on your immortal offering stacks.

I already knew that the top #1 and #2 (with the talent) damage types apply ailments so the rest was just numbers :p

EDIT: You can turn on in game settings to turn on ALL damage numbers instead of only crits (default) so that helps with experimentation a bit but it's certainly no damage meter !