What made my veggie scrap broth turn very bitter?
Both the broccoli stem and the lemon are likely culprits - they probably accentuate each other in a bad way. The lemon (pith) will definitely add a bitter taste on its own - as Spagirl mentioned, the zest and the juice are the non-bitter parts that you would add if you wanted lemon flavor without the bitterness. The broccoli stems are surely not helping either, though - I have added broccoli stems to broth and it was inedibly bitter. Everybody has different bitterness acceptability thresholds, and these thresholds can be shifted for various foods. It's possible you're more accepting of broccoli bitterness on its own than I am, but when combined with the lemon bitterness, it may bring it past your tolerable level. Or perhaps you added too many this time, or the broccoli was more bitter than your previous batch. 

I would definitely recommend eliminating the lemon pith from future recipes to be safe, and recognizing that different heads of broccoli can have differing bitterness strengths depending on growing conditions, and act accordingly (See links below). Everything else on your ingredient list are non-bittering soup ingredients (barring food that has gone bad). Fennel, carrots, and sugar snaps sweeten broths, and onions and scallions flavor them (without making them bitter). 

Link that explains that broccoli grown in hot and dry conditions appears to taste more bitter: https://qr.ae/TDDHbt

And you could blanch the broccoli stems (in case they're a bitter batch) before adding them to a broth: https://www.livestrong.com/article/476765-how-to-remove-bitterness-from-cooked-broccoli/

Edit to add: if you want to use up lemon peels, you can toss them with sugar to make a non-heated, non-bitter lemon syrup. It's incredibly useful and one of the best kitchen discoveries I've made:

https://www.seriouseats.com/recipes/2016/03/fresh-lemon-syrup-recipe.html