It's not true to say that he had no projects in 2013. He had Coriolanus, which he was preparing for from at least the summer onwards and opened in early December, and Thor: The Dark World (principle photography happened at the end of 2012, but a lot of Loki's bits in the final film were from re-shoots in 2013). He obvously had promotional duties for Thor fall in the middle of what would have been Coriolanus rehearsal period, so they started rehearsing ealier than usual, then had about a month off for him to promote his film, then went straight back into rehearsal. That accounts for about half of his 2013!
Films can be bubbling under for a long time before coming to fruition - it's also quite likely that he was taking meetings back in 2013 for ISTL and High Rise, since he was involved with those before the financing came together.
I'm willing to bet the first half of this year, when it looked like he wasn't working, was actually filled with meetings about projects that we won't be hearing about until next year.