That is a strange one, and not sure why that would happen as no changes were made to the email functions. I have had two other developers contribute code for new features, but nothing for the email functions yet.
Version 1.10.0 added a field to one of the database tables, which would have forced an update on that table when activate/initialized, but that was on the sheets table, and not on the signups table. But, perhaps the reminder1_sent and/or reminder2_sent values (there are two) in your signups table somehow got reset to the default value of false (zero). That did not happen on my system in testing, as it should not happen, but I’m just taking a wild guess.
The only way to know would be to look at the wp_pta_sus_signups table on your server (using PHPMyAdmin or other database tool) and see what those reminder sent column values are.
The only change from version 1.10.0 to version 1.10.1 was a very minor fix in the CSV exporter, which would not affect the reminders system, or the database, at all.
I will put out a quick update shortly that will add one extra check when compiling the list of reminders, to check to see if the event has already expired, and skip those. That shouldn’t be absolutely necessary as reminders should have already been marked as sent before an event expires, but it will be an added extra check just in case something didn’t get set right, as appears to be the case for your database.
If reminders continue to be sent on your system after the next update (which should be 1.10.2), then there would be something else strange happening on your system that would require closer examination of your database tables. The only other thing I can think of, if it still happens, is that there was a rare issue a while back with one or two others that were using some type of database optimizer plugin, which was rewriting database tables in an effort to speed up database queries, and it broke the functionality of the bool fields (true/false – or 1/0), such that values were always being returned as false (0) and were never getting updated to true (1). If that was the case with your system, though, reminders should have been getting sent every hour or so by the CRON function.
But, it is also possible, that if you are using some type of database optimizer plugin on your system, then when my plugin did the table structure update for version 1.10, that other database plugin might have reset other table values or otherwise messed things up.