To be honest at first I thought it was because Jaina was more skilled or had more time that she was able to find the evidence that her wards were circumvented. However I discovered that rumors of Lor'themar's issues with Garrosh have spread to the point that the Hozen were hearing about the discord.
So I got to thinking what if Hellscream heard about the rumors and this scenario popped in my head.
Garrosh hears the rumors and decides to ensure that the blood elves remain in the Horde and strike against the Alliance at the same time. As such while obtaining the Divine Bell was the main mission I think Fanlyr was given a side mission in ensuring that Jaina discovered evidence linking the Sunreavers to the theft of the Bell, which leads to Jaina's Purge and Rommath's rescue.
While Garrosh now has to deal with Kirin Tor forces on the battlefield, he was also ensured that more blood elves will be dedicated to the war effort.
However everyone plan is not without a chance of side effects. Case in the point the Sunreavers blame Garrosh for their removal from Dalaran as much as they blame Jaina.