That's true, but it appears February 21 is the "big one":
Code:
date open interest # of contracts, each contract is 100 shares
2020/Feb/07: PUTs: 237,522 ; CALLs: 182,918 <-- these expired last Friday
2020/Feb/14: PUTs: 96,471 ; CALLs: 115,780 <-- this week's expiry
2020/Feb/21: PUTs: 309,790 ; CALLs: 136,461 <-- next week's expiry
2020/Feb/28: PUTs: 23,228 ; CALLs: 29,118
2020/Mar/06: PUTs: 11,245 ; CALLs: 13,336
2020/Mar/13: PUTs: 4,468 ; CALLs: 6,318
2020/Mar/20: PUTs: 214,018 ; CALLs: 126,519
2020/Mar/27: PUTs: 350 ; CALLs: 585
In comparison February 14, this Friday, is a minor expiry. This options series was only created on January 2, when we were well on the way to $450, and most of the open interest was created in the past 2 weeks I believe.
February 21 on the other hand was created in early December, with the stock price in the $330s, when many call writers probably assumed that $400-$800 strikes are lottery tickets bought by fools.
My hypothesis (without firm proof) is that written calls in the $700-$900 range were rolled over to last week's expiry, where the (short) hedge funds who wrote them pulled the Xetra-Nasdaq-Marketwatch stunt to get rid of their weakly hedged liability. A lot of calls (over 100,000 contracts) expired worthless last week - and this week's open interest looks more naturally grown and not nearly as many calls are in the relevant price range, with sufficiently juicy elevated-IV premiums paid to genuine market makers to assume the risk and to finance the delta hedging.
Again, I could be wrong.