The historical significance of Rosh Chodesh Av
The question treated here is not the result of a blind research in Tanach and Chazal about this date. The purpose is to search anything is happening specifically this day; if there is a logical connection between the plowing and the first day of Av.
Gemara Taanit 29b
said משום דריע מזליה דישראל בזה החודש see here
evil... for Jewish people, but the whole month is concerned.
It seems to be not connection whit the 1 Beav itsef (we will see further the opinion of Rabbi Yehuda). The Rishonim on the Gemara don't develop a link between an event and the 1 Av. But the Gemara further report a Berayita which show machloket Tanayim, for the beginning of the mourning custom and for the end. In this Gemara we can find somewhat about Rosh Chodesh Av. For the end of the mourning period, some Tanayim said up to the end of the month and some others up to the fast. For the beginning, some Tanayim said from the first day of the month and some Tanayim said from the first day of the week of the fast. See the text Taanit 29b:
דתניא ... מר''ח ועד התענית דברי רבי מאיר רבי יהודה אומר כל החדש כולו אסור
Rabbi Meir and Rabbi Yehuda thinks that mourning customs begin the first day of the month. But for Rabbi Meir the end of this period is at the end of the fast. For Rabbi Yehuda the mourning period is the whole month of Av.
רשב''ג אומר אינו אסור אלא אותה שבת בלבד
For Rabban Gamliel the whole week of the fast needs mourning customs, e.g. if the fast is Wednesday, the mourning is from Sunday up to Shabbat.
... ושלשתן מקרא אחד דרשו, דכתיב (הושע ב', יג') והשבתי כל משושה חגה חדשה ושבתה. מאן דאמר מראש חדש ועד התענית מחגה ומאן דאמר כולו אסור - מחדשה, ומאן דאמר כל השבת כולה אסור - משבתה.
The days of mourning are learned from a verse in Hosea (2, 15) in which G-d announce that He will cancel the happiness of Jewish people, in "festival", "month" and "week":
וְהִשְׁבַּתִּי֙ כָּל-מְשׂוֹשָׂ֔הּ חַגָּ֖הּ חָדְשָׁ֣הּ וְשַׁבַּתָּ֑הּ וְכֹ֖ל מוֹעֲדָֽהּ:
I will also cause all her mirth to cease, her feast days, her new moons, and her sabbaths, and all her solemn feasts.
--> RM (1-9 Av) learns from feast days, because Rosh Chodesh is also called feast.
--> RY learns from moons, the whole month of Av.
--> RSBG learns from sabbaths (weeks).
Was there anything that happened specifically on the 1st of Av that made it a time to make increased mourning?
Obviously not. Jerusalem was plowed in 9 beav (it is one of the 5 of the list)
Is there a logical connection between the plowing and the 1st of Av?
Yes, the first feast before the fast, the begining of the month in which 9 Beav is.
It seems that Rosh Chodesh av is a Chag as every Rosh Chodesh and the first punishment is to cancel this happy day, of from this happy day. Month and week seems to be quantities (for week there are 2 opinions in 2 beraytot according to Shmuel, one of them does not agree and is congruent with the conclusion of the Talmud).