Giving too much pressure while pushing against the floor, wrong method of doing, working out on the soft floors, doing repeatedly without breaks, fast push ups ,when people overdo when they try out for the first time.
Shoulder pain can be caused for multiple reasons. One possible cause is a torn rotator cuff. If you are unable to lift your arm without pain, this is a great possibility. Another possibility is tendonitis in your shoulder. Both injuries are painful and overuse injuries.
It could be that it is regular muscle soreness if you have just started a workout routine. If you feel more intense pain you could have torn a rotator cuff. It could also be a pec tear near where you shoulder is. If it continues please get it checked out by a doctor.
your rather doin them wrong bro or your shoulders arent strong enough which means you have to train harder and ull get used 2 it if you think your doing them wrong go on youtube search it up look in the mirror if your doing it right