What happened to the Coliseum in the past week?

Too little information, too much speculation. I suggest keeping eyes and ears out in the meantime, I'll monitor my own shard and see if I find any skeletons.

I'm a '4 star' type of mouse. I found at the start I was getting 1000 plus some days and with my 4 star policy I've steadily moved down and now I'm getting 300 per day currency.  It was similar the last era too.  I guess as the era goes on, players invest more on this and that.

Nothing happened

Absolutely no way to prove it currently, but it puzzles me quite a bit considering CG do not offer the means to invest this late into the era, you are capped at where you are outside of the marquee events.

Please do keep an eye out and see if you're noticing anything strange going on in your own shard, CG are never going to confirm if they did anything here anyways. I would call it a marketing tool apart from the fact that there's no way to buy zereks which is keeping us from levelling beyond 95 (or whatever 6-7* people got, 115 something like that).

It could be a way to force people to use crystals on refreshes and maybe that could be what's going on, but yes speculation and just a lot of things that don't add up or piece together currently.

I have no doubt that people invest more into the next marquee if they've missed or had low investment into a previous one.

The piece that doesn't make sense here is that there isn't a marquee event active as of the past week to be facilitating such a dramatic difference in the leaderboards? 

Everyone is capped at where their marquee events got them to outside of the character shards. Zereks (The bottleneck resource) cannot be purchased while the other materials can.

You have probably slowed progress, either you have hit all the feats efficiently for the last marquee or stopped spending ahead of the next marquee. There will be players who are rushing to finish up feats before the marquee changes who were a bit behind and as they catch up there’s more competition. I think it’s why we get these pauses to allow players to catch up so if someone is inclined to spend they have to spend again every marquee.

I noticed it as well, no conspiracy theory.  I posted similar views of my shard in another thread.  I am seeing 5 star players with all toons era level 110.  I am a 5 star player and I was only able to get 1 toon to Era Level 110.  There is no way to earn Zebek material since the challenge mode started, so how on Friday did they suddenly get all their toons to 110?  And I agree, there are names in my shard I DO NOT recognize, but that does not mean they were not always there and just suddenly spent a ton of money last minute since the Rotta Reqs were updated.  Having loaned units below 100 is not good on tier 10 bosses.  And no comms from CG, Meathead appears to spend all his days now talking with a select few in some discord server.