A321 pilot reported during a ground start ground personnel approached the running engine to disconnect a hose.

Narrative

We arrived at our aircraft with an inop APU on a warm evening in ZZZ. Upon arrival we confirmed with ground crew that we would be doing an external air start and reminded them we would need to be efficient with it to prevent excessive cabin temperatures. During boarding; one of the ground crew came into the cockpit and informed us that they had the high pressure cart attached and would be 'starting number 1'. We reminded them that we'd be part of the process and will wait until boarding was complete.Once boarding was complete and all boarding doors were closed the tug operator gave us our standard push back brief and cleared us to start engine 1 while the rear cargo door was open. We delayed the start until the door was closed; but prior to attempting the start found that no pressure was detected at the start valve. The tug operator informed us it may take a few minutes to correct. At this time there was no low-pressure air cooling the cabin and the FA called to inform us that some passengers were beginning to look overheated. We asked the tug operator how much longer until ready because we may need to deboard and he responded by telling us that they were ready. The captain clarified 'we are clear to start 1?' And they responded yes. We noted the appropriate High Pressure (HP) psi and initiated the start. 3-5 seconds later the tug operator emphatically commanded we abort the start using non-standard language. I manually pulled the engine master switch to stop. He informed us that they had forgotten to physically disconnect the low pressure hose and one of the ground crew had attempted to disconnect it during the initiation of the engine start; coming dangerously close to the starting engine in the process. The Captain confirmed with the tug operator nothing had been damaged and no one had been hurt.We were becoming increasingly concerned over the safety of the passengers in the warming cabin and proficiency of the ground crew. We reconfirmed that everyone was clear from the aircraft and we were now safe to start the engine. They confirmed we were clear to start; we started engine #1 with no further issues; completed the push; and departed the gate ramp after confirming with the FA that the cabin was cooling and that all passengers appeared safe.