As I see it - Previously Cummings would have expected to be appointed to the new role of chief of staff. His little daytrip earlier in the year made that impossible though and he knew it.
He tried for the next best option, which was to put "his man" into the job and operate him remotely.
Boris girlfriend (a former senior adviser herself) saw her opportunity to thwart this, with a little help from her friends, and talked Boris into doing something different, which made Cummings realise his influence / authority was fatally wounded.
So rather than hang on to be further humiliated, he jumped ship.
Despite what anyone may think of Cummings, I find it extremely worrying that Boris,s girlfriend is apparently now effectively in control of the Government.
Cherie Blair was far too involved in Govt. business during tainted Tonys tenure, but this seems to go far beyond that.
I wouldnt be surprised if Boris doesnt last until the next election. Either by deciding himself or the men in grey suits deciding for him.
Its pretty evident he isnt really up to the job, but as always the problem is, who is there to replace him with ?
Personally I dont care much about what becomes of Boris. It looks like he will have been effective in getting the job done which was the most important job when he came to office. After January 1st, they can chuck him in the Thames for all I care.