Have you had a lot of rain recently ?
Also, hows the drainage in those buckets ?
Just throwing out a couple ideas, ive had a few plants that weren't draining as well as i thought they were get yellow leaves and drop
Best way I've found to determine if they are draining properly, is to pick them up and get a feel for how much they weigh both when they are thirsty, and right after they've had a good drink.
Can also sharpen a pencil then stab it deep into the soil and look at the fresh wood on sharpened pencil when you remove it, and determine if it picked up moisture or is dry.
Hopefully others will chime in with their thought's and ideas
Ps. your pumpkin/squash looks good Do you eat the blossoms, or just grow for the fruits ?
best of luck to you on your grow