There's a chart floating around here that has a column for pH (rising, falling, and unchanging), EC (rising, falling, staying the same) in the various possible combinations, along with a third column, "What this means." I think it even made the distinction between, for example, a gradual fall and a sudden drastic one. Plants using a lot of water but not a lot of nutrients (ones in hot locations transpiring, etc.) will cause the EC to rise. Stuff like that.
I know it has been reposted many times, but I can't point to a single specific thread where you can find it. Wait, I believe @Rifleman has posted it (along with some other useful graphics/charts). Maybe he will be kind enough to repost it here if/when he gets my "page." Assuming he has a copy handy, of course. And that I'm not completely mistaken about him having posted one in the first place, lol.
Anyway, if you see the chart, it lists a few possibilities for rising pH. Maybe something on there will cause you to think, "Hmm... That makes sense." Or not, who knows?