I heard two things:
and for now I'm chalking up as a rumor until its official
One is EA said cap is 2.2 million XP, and resets at certain time every day (like daily's / dailies?), no time was listed so I don't know exact if that is the case. Another is said EA chat told them it's a glitch that caps to 1.5 million XP and its a timer (either 24 hours from when you hit it OR it resets at 0 or 4 UTC ) / devs know about it.
Also some report closing game completely works, others say it doesn't work, but they uinstalled / reinstalled the game and its golden, and other people said that doesn't work.
Majority have had success with waiting (most say 24 hours or reset after UTC time, one guy said it was 9 PM PST for him)