idk if I should be exposing my own fave but few of the main points that K-Sones are fuming over:
EXHIBIT 1:
"Personally for TY's fans"
Taeyeon has emphasized numerous times that her instagram account was "personally for TY fans" and will be a place of her interacting her fans and with all the "instagram hints/proofs" flying out, fans felt that they were just being played around by Taeyeon. If the posts she left was received as "for Taeyeon fans", now, the fans are taking it as "maybe for us, maybe just another subtle hint to Baekhyun"
Not to mention that she had her private instagram account exposed so all the "couply" things could've really happened there if she wanted to do so
EXHIBIT 2:
-Honeybunnyveryfunny: Sunny's account before she made it public
-This was before Taeyeon exposed her account / made it public
Taeyeon: omg this is so much fun. I don't know if they know me or not but it's fun
Sunny: Follow them back~ International marriage lol
Taeyeon: omg wait. Hurry!!
They are coming!!
The "They" in this context sounds a bit more harsh but I couldn't really find a word to fully describe it
But this was when some
obsessed fans figured out her account and started following. Her calling her fans "they"
in a harsher matter and the fact that she repeatedly said insta was for "her fans" kinda added the fuel to fans fuming
-
I'd say it's more of a "trust" issue than the actual dating itself.
The fact that Taeyeon didn't put much effort into hiding the relationship (unlike Yoona who tried her best to hide) but she wore couple items, hints on IG, driving with an OPEN car kinda made the fans question Taeyeon as well.
There were a few more points but it's too much work
God Korea seems like such a horrid place in terms of entertainment industry. 
GOT7, AOA and HISTORY were nice.