Ian Parmeter, Research Scholar, Centre for Arab and Islamic Studies, Australian National University lists the following reasons:

On the attack:
1. "An important factor motivating Hamas towards violence is that it has to watch its flanks. Other smaller, but increasingly extremist groups, are contesting its authority in Gaza, notably Palestinian Islamic Jihad."

2. The Netanyahu "government has made no secret of its desire to annex the West Bank and has permitted significant expansion of Jewish settlements in the territory, which are illegal under international law."

3. Under an agreement predating Israel’s foundation, Jordan has custodianship of the Al-Aqsa religious complex. Israel aimed to respect Jordan’s role when it signed the Israeli-Jordanian peace treaty in 1994. But Palestinians see the visits by Israeli ministers and non-Muslim tourists as disrespectful of the sanctity of the site and counter to this undertaking.

Hamas has also claimed these visits have led to the desecration of the Al-Aqsa site, an argument obviously aimed at winning support from Muslims throughout the Arab and wider Islamic world."

Why Now:
1. "Significantly, Hamas has named its action “Operation Al-Aqsa Flood”.
This provides some clues to the primary reason for striking at this time, which emphasises what Hamas sees as Israeli acts of desecration of a holy Islamic site."

2.There has recently been strong speculation that Saudi Arabia is about to make its own agreement with Israel.

This is of great concern to all Palestinians, not just those in the West Bank, as it further reduces pressure on Israel to reach a settlement with them."

https://thewire.in/security/why-did-hamas-attack-and-why-now-where-is-the-conflict-headed