Post on an alt because you think it might be funny, or fit because of class / race? Maybe even forgetting which alt you are posting on?
Nothing nefarious, just change who you post on throughout the day for the silliest of reasons.
I have so many alts its hard to pick just one. I haven’t posted on this one in several months and just realized, hey let me check her out…
People dont post on alts cause people are grinding for level 3 forum power
Not really. I’ve used this character for the forums for over a decade, no point in changing now.
I did that at first, but then realized I needed to get a higher tier to post links. So I chose this alt cause I think she looks great, and I love her name.
edit: I also don’t use my main cause people have found me in game and harrassed me.
It’s worth it.
I generally only post on alts if there is a particular thread like “post on your druid” or if someone is asking a monk-specific question and seem to be unhappy with non-monks posting.
Outside that it’s just when I forget to switch back after one of the aforementioned instances.
I switch around to different alts if it fits a post more, yeah.
Some jokes and more in-character comments are more fitting with some alts, sometimes it’s about showing off different mogs, race or class specific threads, ect.
Basically, yes. 
Ya I have no chance of ever getting level 3 since I had a silence so it doesn’t really matter.
Knowing you cant get level also relieves the stress of doing anything wrong. I can say pretty much anything and the worst ill get is a forum silence (or I guess maybe a full ban?) I never said anything horrible but ya know, some people just disagree.
I did that in the dwarf thread 
They said post on your dwarves and I’m over here like
“Hear hear!”
I mostly post on my main, my alts are too undergeared.
No idea why its set to my Mage…but he will go forth posting funny and stupid things.
Time is like that.
I post on whatever character I’m playing the most in game. I’ll never have forum level 3 so I don’t really care about that.
I couldn’t care less about my forum rating, I switch frequently. I tend to post through the eyes of the character I’m posting on, so if I’m going to say something pro-Alliance I’ll hop on my Alliance main. If I need to post something, I dunno, pro-worgen or pro-warlock, I’ll hop on my worgen alt.
Beyond that, I choose my character based on my mood. I’ve been feeling very Horde-y lately, so I’ve been on Dedric. When I’m feeling Alliance-y again, I’ll switch over to my Alliance main.
I only posted once on an alt…my gnome rogue (or if you prefer rouge…
…and that post promptly earned me a 3 day silence.
Since then I’ve stuck to my Tauren for posting on BfA (modern)
On the classic forum I post on whichever of my 8 toons (1 for each class) best fits
the question I have, or reply I’m giving.)
I do that all the time.
In fact, I do many things to make GD laugh
I’ll switch if it something silly like: “Check In All Gnomes!” Then I’ll switch to a gnome, otherwise I just post on this guy.
This is my main…
I do it for trust levels and because I like the transmog I have on him.
Oh, I would never do that.
Yes you would you LIAR!