Jaime abandoned her to go north and join the fight against the White Walkers and the army of the dead at the end of Season 7. This is treason and the fact that he abandons her for "someone else" is probably enough for her to act out of spite and get rid of him to.
JAIME: I pledged to ride north. I intend to honor that pledge.
CERSEI: And that would be treason.
JAIME: Treason?!
CERSEI: Disobeying your Queen’s command. Fighting with her enemies. What would you call it?
Game of Thrones, Season 7 Episode 7, "The Dragon and the Wolf" - Official Script
It's worth noting that Jaime no longer trusts or believes in her anymore which is also probably part of the reason here. He doesn't believe her that she is pregnant and seems to have officially given up on her so whilst she didn't have him killed on the spot she's thought about it more now and wants him gone. Perhaps she just didn't want to watch him die.
CERSEI: There’s one more yet to come.
[...]
JAIME: (quiet) I don’t believe you.
After a long beat, Jaime turns and walks away, right past the Mountain and his drawn sword.
Cersei watches. She does not give the command
Game of Thrones, Season 7 Episode 7, "The Dragon and the Wolf" - Official Script
It's also worth noting that Bronn is friends with both Tyrion and Jaime so it could just be a strategic move on Cersei's part to attempt to get rid of Jaime and Tyrion but also get rid of a potential enemy from her vicinity.
As starpilotsix has said in a comment we can't rule out Qyburn having given the order off of his own back without Cersei's knowledge. In the scene we only ever see Qyburn and a Queensguard member (I think) so he could be acting independently.
I personally doubt this is the case as Qyburn has a lot to be thankful for to Cersei and even more to gain if they win. I doubt he'd throw it away over something like this but it could be possible. In his own words:
QYBURN: When the Citadel expelled me, I thought I would die poor and alone, but in exchange for my service, Queen Cersei made me her Hand. What would she do for the man who rids her of her treasonous brothers?
Game of Thrones, Season 8 Episode 1, "Winterfell"