Likely a phase.
Sleep always went off track for us when my DS learned a new skill so if he learned to count to 20 or backwards from 10 or was trying to write his name things like that would throw his sleep off. Maybe you see your DS trying hard to master a new skill or it could be you don't see it until he actually 'gets it' and his sleep returns to a more normal routine again.
It could however be a change in seasons, not sure where you are but perhaps it is lighter outside now (like here in the UK) and he feels it can't be BT because it is so light? If it is something like that you could try shutting all the curtains 30 min before you go up to bed so that he gets the idea of it becoming night time.
Another big one here is the need for physical activity. Mine has almost always needed a big physical activity just before his bath time to get rid of any last bit of energy. So running, start jumps, press ups, jumping, playing chase. We used to do a Toddler Band every night after dinner, marching with instruments around the house back and forth, he'd end up jogging. If you can't get outside then perhaps consider setting up something indoors like this, or an indoor assault course.
Maybe one of these things will help.
