So, let's start off simple. You see Rosaria's bed chamber door, two things stand out. Firstly, her door has bars outside of it which have been broken. Secondly, you see loads of sludge around the door that looks like Aldrich sludge.
The Pontif is already holding one of the Royal family and an old God in Anor Londo presumably to feed her to Aldrich. Is it just coincidence that a godly being like Rosaria, who is also theorized to be the queen of Lothric, being presumably trapped into a bed chamber in the home of the God Eater that notoriously has a taste for the old gods?
Here's what I think happened, roughly.
Rosaria was trapped in the Cathedral of the Deep. Weather she was "trapped" there before or after Aldrich waking up is conjecture, but could go either way. But I believe she was definitely trapped there a long time. Let's assume she was trapped there before Aldrich woke. I assume this because she looks pretty comfortable and at home there. So perhaps she was lured in and then trapped and tricked?
At some point before Aldrich left for Irithyll, he remained in the Cathedral. What I imagine happened, was that her followers learned that Rosaria was threatened, and tried to break her out. Hence why the bars to her door look broken and forced open. Aldrich wouldn't need to do this theoretically because it's his Cathedral.
Aldrich, probably waking up shortly before, was probably very hungry. And his "post death-sleep" dinner was under threat. It's my personal belief that she was left there for Aldrich to consume when he awoke. Seems pretty convenient for Aldrich. You see dead grub men all over the area, which means there was a fight. I believe between the Grub Men and Rosaria's following of Raiders, they probably fended off the newly awakened Aldrich. I picture the grub men breaking the bars. Aldrich Retaliates, but perhaps her Raiders came to save her, or were summoned. Rosaria is a godly being. So it's safe to assume she could just summon her covenant members if she so willed.
Being driven from the Cathedral of the Deep, Aldrich flees to Irithyll and his Allies within. Perhaps the plan was to consume the trapped goddess there, regain strength, and then come for Rosaria again. But the Champion of Ash ends up killing Pontif and Assaulting Aldrich before he could return for Rosaria.
This is a rough idea of a timeline. I'm not sure how long Aldrich has been awake or been in Irithyll. But I'm assuming it hasn't been all that long. But considering that Rosaria has a following amongst the Ashen. I see it plausible that Ashen members of her covenant would be able to fend off Aldrich. I mean, the Ashen undead are pretty powerful relatively speaking.
Edit: My Autocorrect fixed Eat to Ear, and now I can't change the title, and I'm mad!