The Palestinians planned to take advantage of the normal high prices of vegetables during the shmitta year in Israel. With the Palestinians in Gaza being the main supplier of vegetables to Israel's "yevul nochri" market during shmitta, they expected to rake it in with high prices, similar to what happenes every shmitta year.

To that end the Palestinian farmers in Gaza planted much more than normal, specifically cucumbers and tomatoes but also other vegetables, expecting to make that much more money. They then sat back and waited for the 30% increase in prices. They were going to take it straight to the bank.

Instead prices dropped.

According to NRG, most yevul nochri shmitta produce comes from the Palestinians in Gaza, with some coming from Judea and Samaria and a bit from Israeli Arabs.

How did they flood the market with vegetables to the point that it forced prices down?

It turns out that the way it works is the distributors buy x amount of produce that they need for the shops selling yevul nochri. The remaining produce then gets sent to Judea and Samaria to be sold there (among the Arabs). There is no direct connection between the Gazan farmers and the wholesale market of Israel, but after being transferred to Judea and Samaria it then makes its way back to Israel's market. And that is in addition to the produce grown in Judea and Samaria.

So, basically, they flooded the market. They overplanted and then sent all the produce to Israel, in both the direct and the roundabout way. And the prices dropped.

Israeli consumers (who buy yevul nochri produce) benefited from rare normal prices during a shmitta year, but the Palestinians frustrated themselves and did not make the profit they were expecting.

Another factor was that they could not export to Russia, due to the drop in the ruble and the market there was also flooded.

I don't know why the Europeans don't buy vegetables from Gazan farmers. They could support the Palestinians they love while getting relatively cheap produce.