Air Carrier Flight Crew reported Low Altitude Alert during descent for landing.

Narrative

Below minimum altitude at UTEEE waypoint.Nearing the end of the LAIKS STAR into AUS for [Runway] 18R; descending with VNAV engaged; Approach was busy and asked us to expedite our descent. We were complying and all seemed normal other than the expedited descent. Approach cleared us to fly the ILS [Runway] 18R from HOUKM so I; PF (Pilot Flying); set the FAF altitude of 1800 [ft.] in the MCP (Mode Control Panel) altitude window. But with short notice; Approach asked us to cross HOUKM at 3000 [ft.]; for some reason; rather than the published 5000 [ft]. I typed that into the LEGS page; and it was doable; but required us to hustle down; so I used gear and speedbrakes. At some point; I must have selected Level Change (I do not remember doing so; and Level Change would not have helped or made any sense; but there is no other likely explanation). I believe my FO (First Officer) was also unaware that I selected Level Change. We were slightly above the crossing restriction at HOUKM; but Approach gave us relief for that. I then continued descending; still unaware I was in Level Change and unaware I had no altitude protection for the 2800 [ft.] minimum at UTEEE (1800 [ft.] was still set in the MCP window). At about 2000 [ft.] and prior to reaching UTEEE; Tower issued a Low Altitude Alert for us. That was when I realized my error. We leveled at about 2000 [ft.]; intercepted the glidepath; completed the approach and landed without further incident.Better automation management and awareness on my part. Approach should have given us the published altitude at HOUKM or an earlier change to that published restriction.