I have an interesting theory:
When Kate first arrived in Ramsey Street, she got together with Declan. When that relationship, she rebounded by going on a date with Kyle.
Later on, she gets together with Brennan, but then he has to leave town and is confirmed dead. This is truly "devastating" for Kate, and her only way of coping was to go off and kiss one of her students. Now this resulted in her being temporally suspended from teaching. This was too much for Kate to handle, so she dealt with it by going on holiday and having a quick relationship with a random guy she met.
Later on, she gets together with Mason, but that breaks down after she finds out he committed a robbery, and her ex-boyfriend returned from the dead. Now, her breakup with Mason was also "devastating" for her, and her only way of coping was to sleep with her best friend's boyfriend.
See a pattern emerging here?????
Now, currently Kate is starting to get together with Kyle, but neither of them are aware of Georgia's pregnancy. Once Georgia's pregnancy is revealed, it could destroy Kate and Kyle's relationship, which would be truly heartbreaking for Kate. And coincidentally, it could be around the time when Brennan returns, and if history is anything to go by, she will end up back with Mark in no time at all.
I could be wrong, but that's my guess.